I know, metroidvanias usually use a side-view, but if i still use all the mechanics that make it a metroidvania, is it fine?

a Metroidvania is a game with upgrades and a big open map. it does not state once it has to be a platformer

Metroidvanias have been made as first person shooters, so too down probably won't be a problem.

The perspective doesn't matter. Generally what makes a game a metroidvania is exploration linked to character growth in an semi-open enviroment. Backtraking is a common staple of the genre. See the links on the jam page, they will help you design your game.