Others have said this but the pixel art and animations are really good. However the screen is way too busy. That fast moving animated colorful background, while nice looking, significantly contributes to player confusion. I can already barely see those tiny bone attacks, but add in those distracting beautiful and colorful stars and my mind is exploding with too much stimulation.
Also, It would have been nice to have seen how much health the player has, just so the player could know how well or badly their doing at a glance and hopefully give those health power ups feel a bit more impactful.
The juice (particle effects, sfx, explosions) is just the right level for this type of game, which is really important for this genre so good job on that.
This game does a lot of things right so don't let peoples emphasis on the one visible flaw make you think it wasn't good. The flaws in your game could be fixed in a few hours. Which means you made a really good base that just needs a few kinks ironed out. Good job!