Should I submit an art portfolio to MIT?

If it does no harm in submitting, submit it! It can be seen as an extracurricular/passion which is what MIT wants. It will definitely help your application.

Submitting an art portfolio, or a portfolio of any kind, to MIT can be a great way to show off your other skills and interests which you might not typically spend much time discussing in your application. So while an art portfolio might not necessarily be applicable to your application it can most certainly be complementary to it.

Let's take a look at MIT's website and see what they have to say about portfolios: https://mitadmissions.org/apply/firstyear/portfolios-additional-material/. So, it looks like you can submit up to 10 pieces of art. Personally, I would aim for as close to 10 as possible with the important caveat that the pieces are not incredibly similar. If you did a series of paintings you could maybe include multiple pieces from the series but I wouldn't include every piece. It's probably better for you to submit pieces that demonstrate a variety of mediums, concepts, sizes, etc. Besides the artwork itself I think one of the most important things will be to make sure you will be able to explain your inspirations for your work or provide strong descriptions of your pieces. One last thing. The MIT website mentions they welcome portfolios from "Creative individuals with exceptional talent..." and you said your work is mediocre. While I hope that's you just being tough on yourself and your work is above mediocre, it's something you'll want to think about before submitting your work. Art is obviously subjective but if you truly think your work is mediocre and you think you might have difficulty describing your work or explaining your inspiration for it might make sense to re-think if you should submit anything.

Overall, I think submitting a portfolio can be a net positive in most situations. MIT seems to be a school that actively encourages students to submit a portfolio. I think in your situation specifically, since you're applying for a STEM related field, an art portfolio can be a really nice complement to your application. The biggest thing you need to consider is whether you think the actual contents of your portfolio are strong enough to be worthy of submission. I think the situation where this might hurt you is if your artwork is average and you don't bring anything interesting to the table with your concepts/descriptions/inspiration. Even then, it might not be too much of a bad impact, especially if you talk about your passion for art elsewhere in your application.