Basically what >>10788
said. If it lacks effort or basic understanding of how colours work, it becomes very unpleasant aesthetically to me, enough to make me not want to play the game.
Well to say something different, it can have good graphics in general but if I don't like the art style, I pass.
this is such a broad question you could write "extremely bad" and it'd still be a valid answer
there are… a lot of fangames that have really bad graphics in my opinion, or bad design in general that make it hard for me to want to play them. my advice to aspiring pixel-game-makers is to take a few months at the *least* studying pixel art, doing a few one-a-day studies, etc.
Yume 2kki does this pretty often in some places like. Sign World, Bug Maze… they're all pretty "eugh" to me in terms of art.