No offense intended! From the description of the MIL I am picturing more likely probably someone in her 80’s but I didn’t want to assume! Everyone is different of course, but the 80 year olds I know in my life as not as sufficient or tech savvy for traveling!
You go CC 70+!!! 
It’s funny, because our family has a dry sarcastic way of communicating & I could totally see someone texting any of that stuff, but the internal read would be different, we would all know it was basically said with winky snarky face, not desperately needy, not negative. It’s all in the tone, but texts have no tone, so the tone is your personality.
Until 2 years ago my mother (now 89) was traveling on her own. Tech savvy, no, but she managed to navigate airports and used her cell phone (not smartphone) to contact us if there were any issues.
My inlaws are 85 and 90. Both travel, use IPhones to text and FaceTime and are computer literate. They would be insulted if anyone suggested otherwise based on their age.
I have a relative who sounds similar to @Hoggirl MIL. She comes across as negative but she is actually anxious and that is how she works through her anxiety. At the expense of all of the rest of us but I’ve learned to cut her some slack.
My 96 year old MIL bad mouths her younger friends that are not as tech savvy as she is. MIL can text, email, fax, play online bridge, use apps, including using Uber often, and Netflix! She will say her friends are stuck in their ways and will not even try to learn new skills.
While my MIL is not typical for her age, I hate listening to her complain about those in their late 80’s and early 90’s. But my critical MIL would be a whole different topic!
I’ll take back my comment re: age and travel/cell phones! My 87 year old mother can barely use her flip phone and would never in her lifetime travel alone! There are those elderly out there too!
I’m happy for your family members who can travel and be tech happy. Please don’t make this thread about my not meaning to be offensive comment. It was meant to be complementary to OP’s MIL who was traveling on her own!
You must be married to my husband’s long lost brother, because you just described my MIL! She wanted a smartphone, learned how to use it (although not as well as your MIL). She’s finally slowing down at 96. Pandemic isolation has not been good for her mental acuity.
Comments since she arrived:
Last night: “I feel guilty that you are doing everything for dinner but not so guilty that I want to help.”
“What’s that step stool for? People with short squatty legs?” (Just so you know, tall = superior.) I am only 5’4”. I’m not sure how tall mil is now - she’s shrunk some - but she is “tall,” and I am not now nor have I ever been.
Walking on the beach this morning:
“Don’t worry, I won’t walk down the beach looking like that! I’d cover up.”(said too loudly about a woman walking on the beach in a swimsuit - thank goodness for wave noise and wind)
“That poor woman walking in front of us - she’s knock-kneed” (also said too loudly - again wind and waves are a blessing) Multiple complaints of her new sunglasses sliding down her nose.
At lunch: I’m just going to sit here on my fat a$$ while you wait on me.” I’m sure this is supposed to be funny, but it’s rather annoying. I’m sure it will be more so after a week.
At the pool this afternoon:
“They haven’t RSVP’d to the party, but what you can expect - they are Polish.” (!! - double exclamation points are MINE!!)
Multiple complaints about how she looks in her swimsuit/how fat she is/ how large her stomach is. I don’t what I am supposed to say to those comments???
MANY of her “observations” are about the physical appearance of other people. She said three times how unattractive Frances McDormand (won Best Actress for Nomadland) is.
She had about half a bottle of wine at the pool. Don’t worry - no glass BY the pool - our condo has a small, community party room with a fridge next to the pool. She did stop drinking wine when she came in.
She talks and talks and talks. Constantly. It is just a bit wearing.
Thanks for “listening”
Oh my! My dad would often make loud comments about other people within earshot. Very hard.
Hope you can get in some alone time every day to maintain your sanity!
Forgot to write - my mil is 82. She says that a LOT as well. She is fairly tech savvy? Not great but not terrible.
Oh geez, it’s the continuous little comments that are grating. Sometimes you wonder if they are jabs (like the short leg comments), or if they are just thoughtless. The “Serving” comments are another. I don’t know your MIL of course, or the things she says around your husband, but for my MIL those would be meant as passive/aggressive statements. Enough to get in a sly dig, but if you act bothered, she was just “teasing”. Why so defensive?
Mine is 80 this year and I’ve noticed an uptick in an obsession with other’s looks. Plus she’s always begging for compliments and affirmation, which I find hard to give to her anymore. Same sort of statements as yours does.
Are things better since yesterday? When does she leave?
I think this here is the best thing you can do to deal with the comments - telling us about them!
Are you counting the times you’ve bit your tongue? I’d say you are well on your way to earning a treat, if you are playing my game! 
I wonder if it’s generational, this emphasis on other people’s looks. My late MIL also had that. Which was quite unattractive!
I think these comments are thinking out loud…very common. These might be comments others of us might think but wouldn’t say out loud.
Re. The swim suit and how she looks in it? How did you keep from saying “would you like to go shopping for a coverup?”
Yikes.
Has she always been like this? If not, you might want to consider that fact that the loss of a socially-appropriate filter can be an early sign of the beginning of cognitive decline. That definitely happened with one of my older relatives – things he probably had thought for years but which now came out of his mouth.
So is this at least the count down to the end?
@conmama and others
It’s a bit better today. She said she just wanted to loll about, so she is out on our back patio reading while I am inside puttering around.
Still with the negative comments. I do think some of them are “jabs” at me and a bit passive aggressive. This morning she complained about the tops of her feet being sunburned. “I guess my legs aren’t short enough for me to reach the tops of my feet to put sunscreen on them.” 
Honestly, typing them out helps, because I can “see” just how comical/silly some of them are.
I really don’t know if she has always been this negative or not. My fil was HORRIBLY negative, critical, mean-spirited. So, if she has been, it was unnoticed by me by comparison because he made no attempt to conceal his disdain for others. She has definitely gotten more negative since he died two and a half years ago. He was also fixated on physical appearance. And, I do think it is somewhat of an age/generational thing. He met her because he saw her walking in their downtown and thought she was attractive (and she was TALL and he was TALL) and followed her to her office. She will jokingly say that he thought she would be good for breeding purposes. And laughs. And, I cringe and think how that was STALKING and gross that is.
When we would watch old movies of them, he would say to her, “Why don’t you look like that anymore?” You know, from something that was 40 years prior. As though he hadn’t aged. So, I’m sure she felt valued for her looks and those of course diminish for all of us. It’s really kinda sad.
Two nights down, five more to go. She really isn’t being unpleasant TO me - it’s just a lot of rambling, observational negativity.
She has a cover up. She is just not happy with her weight right now. Saying that once or twice is fine. Saying, “I would feel better if I lost _______ lbs. But, she belittles herself, which is different to me. Assigns adjectives that are negative. Which is what my fil did to her constantly. One time I witnessed him reach over to her and “flump” (as my mama would have said) her stomach and say, “_________, when is that watermelon going to be ripe?”
So, it’s no wonder she has an obsession about weight/appearances, etc.
Your former fil sounds like a peach.
Many women have a unhealthy relationship with their body image. Add to that a spouse who tries to intimidate you about your appearance and it’s hard! It sounds to me that she heard these negative stories in her head for so many years that she can’t let go of the feelings. So she continues to tell herself the same things her husband did.
And she is transferring his negativity to you.
I personally don’t think that saying whatever comes into your head is necessarily a sign of dementia. I think that as you get older, the filters that you used to have aren’t firing as well. My mil is fine, she’s not losing it. She is unfiltered.
My mil is very careful about her weight, I told her once that when I turned 80, I was going to eat whatever I want. She told me that no I wouldn’t because I would want my clothes to fit. She has also told me I carry my weight in my stomach. She wasn’t wrong but I would rather she hadn’t noticed.
My mother lost much of her filter at about 80. She always said she was happy when I called because she had someone to complain to. Way back, when she and my father returned from a cruise, they would complain for three days. Then they would start planning their next cruise.