The first two games have a very consistent Disney-like style that was common Lucasarts adventure games of the era and earlier. The third game is flashier and more stylized take on the same thing, but ultimately still looks fairly mainstream American animation, a bit more stylized, just more like Don Bluth having a very bad day or Nickelodeon with a huge budget than Disney. The fourth game is essentially the same style as Curse, just in 3D - and it was clearly a bad decision, aesthetically, to go 3D - the tech wasn’t there. Tales, which isn’t exactly a sequel (specifically my understanding is Gilbert regards this as Monkey Island 5, not Tales), is a more successful attempt at essentially the same look as the fourth game.
Ron Gilbert really underestimated the artwork direction Steven purcell(Sam and max) brought.
This is probably the George Lucas star wars prequel effect in action. Some people NEED editors and feedback. After playing Thimbleweed Park, Im convinced Gilbert is one of them. I have no doubt the backlash against him will only get worse as people actually play the game ala the star wars prequels.