The theme was loosely integrated into the main mechanic.
The game was definetely fun. The choice of main mechanic was good and skill based with good audiovisual feedback.
The art was all over the place but in a consistent way. It kind of reminded me of Nubby’s Number Factory. Could still use some polish to be more uniformly weird. Just some general pixel art do’s and don’t would benefit. Like a consistent (or smaller) colour palette to give it that cohesion.
I did not really feel like there was currently a good reason for the overworld/map existing. I think it would have been a tighter experience if you could simply choose a difficulty to fight.
The cheering sound did not do it for me. But the sound design for the combo gameplay was solid. Perhaps an accelerating tune that follows the acceleration of the ball would make it even better.
Check for typos. You had one in the tutorial. I also think there was no need for a tutorial beyond maybe a flashing space-bar in the first fight since it’s quite self-explanatory.
Overall I think you did a good job!