Nice game overall but there are some issues.
First, the jump of the character, especially in 2d, feels very floaty. The jump height is also, in my opinion, a bit too high. Controlling the character doesn't feel like you are controlling a character, more like something static. Giving the collectables some kind of animation in 2d would have felt really good, even if it was very simple. No audio, especially no sound effects, makes the game too silent. Also, in 3d, the fixed camera is pretty bad to play with, especially in the sixth level. Adding some traps here and there will also help the experience.
I know this is a lot of criticism, but your game will become much better to play with these. I know it all takes some time, so take it slowly. Good job!