This kept to a competently implemented simple idea, but I'm not sure it was the best implementation of "less is more"; like, even a low amount of ammo might be a better trigger for a higher score in a scoring system, since it inherently makes things get out of control faster and thus has that risk/reward push that is more abstract than "oops, I took a hit, guess the game's over".
Art was fine, but maybe the player could be a bit more distinct from the enemies & background.