a friend and I have run 100% multiplayer, but I can't seem to find a category to submit it under. Is there a chance somebody could create one?

I think the category will not be added here but try on the category extensions boards : https://www.speedrun.com/nsmbwmemes/

(Btw you need a video to add a category)

I’m guessing since not many people would run it that it would become a category on the category extensions page. And even if it doesn’t become a category that doesn’t make the run invalid or anything. You don’t need to submit it to a leaderboard to run it.