Nice work! I found the controls pretty good, and the bouncing off enemies was satisfying. Clearing enemies to open the door feels like something you don't see very often in a platformer (if ever).
I did find the game began to drag quite quickly though; not enough interesting things to do, and the difficulty was very flat. By the time I beat the sumo guy for the first, I was pretty much done. I had only died once when I wanted to check to see if water kills you.
I think the game would be much stronger if it was very short, with very hard levels requiring precise bounces and jumping to complete (kind like a Super Meat Boy, or Celeste type game).
I was pretty bummed to find that the enemies and pickups reset if you leave and enter a room, making the "score" basically meaningless (you could farm a single set of room for hours if you wanted to). I would much prefer a small number of pickups that are tracked and given a total at the end of the game.
Music was great, if not a little repetative.
Art was good. Nice and bright with great color choices, although the level after the first Sumo boss felt like not a great choice of color.
There is some little stuff I would also tweak:
* Lower he ninja a pixel or 2 so the feet touch the platforms when standing/running.
* Don't place so many floating platforms at exactly the max jump height. It makes the game feel unnecessarily clunky, where the player is constantly hitching on the edge of the platforms.
* Add sound effects, especially for pickups.
