Okay here are some things:
- Squash and stretch could make your animations even juicier especially with your artstyle, don't have to go overboard with it, a moderate amount should go a long way.
- The boss in the end just kinda appears out of thin air after dodging whatever was being thrown at you, idk if its supposed to crash down or whatever but doing some sort of animation to make sense of what it is and where it came from should help.
- Some textures like the spikes seem a little too plain and felt a little too out of place compared to the character, i think just outlines like the main character would work well.
- The hearts could use a little animation (the squash and stretch i mentioned earlier could work just fine)
Also just another suggestion, unrelated to the rest, I think the boss fight could use its own music, cuz at first i just thought it was like any other level, also an introduction of the actual boss before it attacks you, or some way to find out that you're facing an unusual threat