Most metroidvanias are 2d, and some people consider a requirement for the genre, but 3d metroidvanias aren't unheard of and you could definitely make one for this jam!

Either are acceptable, but after 2.5d, you risk people not really considering the game to be a Metroidvania. There was a 3d entry from 2 MVM's ago and it was a fantastic game, and I gave it 5 stars in every category except for metroidvania. I think I gave it a two at most, because it felt more like an adventure/puzzle platformer.

My advice is to choose what you want to learn or practice and complete your game. After that, learn from the feedback you receive and remember the most pointed out criticisms and try to not repeat them in future projects.