Cute concept!
The art style is charming, though the player character looks jarringly out of place.
Writing is fun and, although brief, gives me a sense of humor and playful setting. I enjoyed the random ending text. Perhaps expand the concept to cheers/jeers from the aliens in the stands?
Boss pushes player outside of the visible screen area and it seems like I just have to wait for him to attack to get past into a place I can see the character.
Gameplay feels a bit simplistic, but perhaps the choices aren't telegraphed enough to me as a player. I tried the first boss a few times but couldn't beat him. It seemed like my only choice was to just hit him and avoid the projectile now and then, but his health is quite high so I couldn't kill him before death.
I like the powerups. Could be fun to have some theming for that, like aliens throwing down treats for their favorite gladiator.
The intro screen should either not play or be skippable in entireity on replay.