I like the art style, choice of music and story, but the enemies aren't compelling, the bosses are tedious and the areas are too big for the amount of interesting things in them. I think you should consider cutting some rooms and focus on making sure each part of the game is new and interesting in some way.
I found two bosses, the rabbit, and the pair of kirby trees. The rabbit has the issue that it's easier to tank the damage for the pink enemies than to try to avoid getting hit. They just spawn too quickly. The kirby trees seemed to have too much hp. They stopped being interesting to fight halfway through the battle. Also, if you're not going to use pixel-perfect hitboxes, I recommend you use multiple objects for enemies that are large and non-rectangular. in the case of the kirby trees, perhaps the trees themselves could have just the trunk for a hitbox with an invisible object covering the foliage that relays any damage taken to the trunk.