NSMB's art style is just painfully bad.
I think Nintendo did some "lowest common denominator" approach with it, as to offend no one, but as a result the game looks atrocious.
The tubes and characters are all plasticky and shiny like some early 90s render for a magazine or something, which conflicts with the objects of the game that seem to be lifted right out of the original SMB game (blocks), every "texture" is a little pattern that repeats a gazillion times (see: the grass/ground), and the backgrounds are just painfully boring.
I know not every game can look like the latest Rayman, or Kirby Epic Yarn, or Yoshi's island, but we've also got games like Donkey Kong Country Returns which has a fairly straight forward art style but still manages to be absolutely gorgeous.