I am applying to 8 Early Action Colleges and all of their applications are due this November.
I am finished with 4 colleges applications and supplemental essays and 2 are half way thru and 2 I didn’t start. 
Are these Supplemental Essays weighted a lot during the holistic admissions process.
I am applying to mid tier and a bit top tier schools with a major in Mechanical Engineering.
Thanks! 
I think they can make a big difference. My youngest got into some quite reachy schools for his grades and scores. I know at U of Chicago they thought his “Why Chicago” essay was pretty funny because they mentioned it in a holiday card they sent him when he got in EA. His extra essays for Tufts were also unique to the institution and I think really gave the readers a sense of what he is like.
@mathmom - Is it as important or as significant as GPA, SAT/ACT Scores, or the main Common App essay?
Would it cover up bad test scores or GPA?
Nothing covers bad test scores & a bad GPA. Just write the best essays you can in the time allotted. No changing the rest now.
No one can tell you that. It varies from college to college and from reader to reader. Your best guess will be whatever the common data set considers most important. Here’s a typical one: https://registrar.princeton.edu/university_enrollment_sta/#comp000048a59a9e00000006304217 If you look at page 7 of Princeton’s Common Data set you can see what they consider most or least important. (And notice that they have everything you listed as equally important.)
@mathmom - Does that include the main common app essay as well as the supplements or just the main essay?
They don’t say, I assume all the essays are important. The supplements are there because the colleges want answers to those questions. Don’t skimp on them.
A good supplemental essay won’t make up for subpar grades, test scores etc. But a poor supplemental essay can hurt an application.
You need good enough grades to get in the door. After that everything else can make a difference, but yes subpar stats will not help you. A mediocre essay will be the nail in the coffin. Your only chance with subpar scores is to hope everything else is top-notch.