No don’t ask a teacher for a LOR when you got a “C”. I wouldn’t even ask a teacher I got a “B” from for a LOR.
All of my LORs were from AP teachers I got “As” in.
Well, I guess all the kids that only get Bs and Cs are out of luck.
Maybe the people who gives you Bs and Cs are the honest ones.
The only school my kids applied to (so our of 15 schools total) that specified that they wanted a STEM and a humanities letter was Harvey Mudd. Now… I think for top schools especially, if you can get excellent letters from varying areas, that strengthens your application (shows you are not a one trick pony). But better to have strong letters than varied letters if you have to choose.