I've been looking through the categories of both Base Game and Category Extensions, and I couldn't find anything regarding Pico-8 Celeste. Is there anything regarding this, and if there isn't, would it be possible to add a category regarding this part of the game?

You can submit Pico-8 Celeste runs to the CELESTE Classic leaderboard. Runs using the easter egg version inside Celeste are allowed there as 'emulator'.

Thank you :)