Art design, player animation and sounds are on point, really neat! Even though the boss has no special attacks, it would have been nice to have him be animated somewhat (at least a walking cycle)
It almost feels like you spent too much time perfecting the player animation (which is basically flawless!) that you didnt get around doing much else. That comes down to basically never need to jump (unless you want to hear the cute sound and see him hop) because there is just no mechanical need for it.
Also, with only the "boss" as an enemy, it doesnt truly feel like a boss - you need the build-up of smaller enemies as to warrent calling him a boss imho.