Without proper training, what looks good may be trash. I always thought pixel art generated by diffusion models looked damn good. Then I started watching and reading reviews by actual pixel artists, and all they saw was flaws. And it wasn't just nitpicking, it was things that were fundamentally wrong, difficult to fix and would look awful and amateurish and distracting to the player in production.
Much of this comes from the fact that, as is true for almost everything, an LLM (generative model etc) presents itself as an expert. It'll very confidently produce results that, to a layperson, look quite good. But the more of an expert you are in a field, the more apparent the cracks become.
AI pixel art looks particularly bad because most users don’t even go through the effort of downscaling and then upscaling it using something as simple as nearest-neighbor scaling, which by itself will squash out a lot of high-frequency noise that manifests in the form of terrible looking "fringing". Proper grid alignment also makes a big difference. It’s not perfect by a long shot, but it helps.