What a diet of drinking alcohol 3 times a day does to a MF
I agree. His PC-98 art was decent and looked better than some of his windows era art, but that can be attributed to how pixel art tends to look better than traditional art. That can be attributed to how pixel art is easier to make look good if you're a novice. Take me for example, I don't train my art skills because I am lazy (A kettle calling the pot black), but my pixel art drawings look a lot better than my hand drawn/digital art due to how easy it is to make a decent pixel art drawing when you're a novice.
That being said, when you compare the art ZUN makes between windows era games with each other, you start to notice things like how he started shading less, characters gained faces that are too similar to each other and the drawings are less colorful and more plain. Nitori might be the best example of this. See how less colorful and detailed she is now?
My suggestion and easiest way and most reasonable way to solve this problem: I lock ZUN in a room for 30 days with no alcohol. He will be forced to train his art skills with YouTube videos. I'll only give him crayons and pencils. He will improve in no time.