Baba is You came to mind for me as well, as a game with incredibly simple graphics that still looks _really_ good. I think a big part of that is the consistency of the design.

The high-contrast simple design also facilitates the gameplay. There's no visual noise, so it's very easy to 'read' the levels and focus on the solutions.

With that said, I think the art is also deceptively good. I don't think I could replicate it quite as well (from memory) even having played the game a fair bit, and I certainly don't think a toddler (or lesser hyperbole) could capture the charm of some of the sprites.