I love the simplified color scheme you chose for this game, it really fits a short jam like this. This is a really fun concept and your character feels good to control! You've got good jump buffering which is important since if you stop moving you have to reset. Jumping with the shell feels really satisfying once you get the hang of it too!
Two pieces of feedback: I wish the shell jump had been taught in game. I spent a long time trying to get past the laser in the third level and I only got past because I read another comment about the shell jump. I'd also suggest since the main mechanic is you can't stop moving consider having the player character always be moving and the A and D keys change direction since you have a dedicated button to standing still. My hand got a little tired being force to hold down the movement key or need to restart the whole level.
Awesome job!