Jumping feels bad (too weighty, I'd say), and the visuals are incredibly bland to say the least. I appreciate the fact that all of the terrain is semisolid simply because all of it looks the same and smacking against an unexpected wall or ceiling would suck.
Maybe I'm tripping, but it also feels like the green fireball(?) hitbox is slightly larger than the sprite since I constantly misjudge just how far I need to jump and just barely clip it on the way back down.
The lack of music is also another thing, but please for the love of god don't tell me you intentionally let the game be played in portrait on mobile, it makes for an unfathomably worse experience. (The buttons are also too small + too far into the corners in landscape, imo)
Yippee edit time: Referencing both portrait and landscape screen orientations *should* make it abundantly clear that I was playing this on my phone. The onscreen controls don't even exist playing it in browser, but also the jump is literally fine playing this way. Maybe try looking into that, as holding jump for the same amount of time should yield identical results regardless of input method.