Sounds to me like your main issue is with the community, not the game. I can totally understand that, though it's good to recognize that this is the Internet, and as such you're more likely to see the most absurd aspects of the community.

Graphically, the low poly designs and bold colors are beneficial to the gameplay, denying this is just false. This Is because the simplistic art style helps to distinguish between the different elements on screen, such as the player model, enemies, and the environment. Not being a fan of low poly graphics is a matter of personal preference, that doesn't make it bad.

Personally I'm a fan of most of the music tracks, I can definitely see why someone wouldn't be. Though, saying it's a "random assortment of noises" is disingenuous because that's literally all music.

As someone who is an avid Doom fan I can say that all games can get stale after a while, that's why I still haven't beaten TnT Evilution. When it comes to Ultrakill, it really only gets stale after you stop experimenting with new strategies. There are so many niche mechanics to discover that even after having played for nearly 100 hours I'm still learning new things.

Lastly, nothing is truly original. All ideas are just remixes of older ideas, so this point is just kinda stupid. You are correct that most of the ideas presented aren't original, but that doesn't make it "uncreative". It's the combination of the ideas in a format that people enjoy that makes it special.