Oh man, you really touched me with that ending. Especially that song. I forgot for a moment that even game jam entries can have some soul in them. And I love the "princess kisses hero after being saved" trope, lol.
From a gameplay standpoint, I felt that the triple-shot gun was markedly worse than the default gun. Maybe it deal more damage, but it's not communicated with the game feel. It was annoying sometimes that I'd have the goated yellow laser gun and then out of nowhere get the triple-shot gun against my will.
I felt that enemies either spawned too close or moved a bit too fast for me to react while moving away from the final boss. It make me take some cheap hits, but at least there was a big enough supply of health powerups that I didn't get at all close to dying.
And apart from that, the usual "would be nice to have more enemy types and gun types" etc yada yada.
Obviously for a jam entry it would be impractical to have the player actually play through all 255 levels first... but I'm kept wondering how awesome this game would feel to beat if it was an actual full release like that. With even more fourth-wall breaks at the final boss fight and ending. This concept certainly has potential.
But please clarify... how do you see a "loop" in this game's backstory? So all other players just tried to get to level 256 and beat it and simply lost? Game-overed? Can you really call that a loop? Or was there some kind of glitch that didn't allow them to reach level 256? If so, how did we overcome it? Your interpretation seems to make sense, but under scrutiny, I don't know.
Either way I really liked this so don't let my criticism get too in your head lol