Not the answer you would like to hear, but you should make the games you want to make. If that game is a platformer, go for it! I don’t know if this would be your first game but if it is, I would suggest entering some game jams first. That way you can get a bit of a feeling of how it is to finish a game and gain some exposure (if that is something you would like of course) and feedback in the meantime. But to answer your question a little more specific, sure platformers can do well on itch.io. But then again, any game can do well if it’s fun and creative. Itch.io is just a platform after all and there are a lot of different types of people on here who play and make different types of games. I think you will always find an audience if you persist long enough, but should not necessarily be your end goal. Just have fun making games and just go for it!