I don't hate Sunshine but it's not a proper Mario game because it features a unique control scheme and abilities that were never seen again. Mario doesn't control like Mario except in a small subset of levels. Compare that to Galaxy and 3D land, which both control pretty much like Mario 64, and Sunshine is definitely the odd one out.
It should be noted that while Pikmin is nice and Animal Crossing is fine, the Gamecube lacks powerhouse debuts for franchises (most of its top games are just iterative) and doesn't have many huge culutrally defining hits like Mario 64 or Goldeneye from the N64, or a billion games from the NES/SNES era. When you think Nintendo you think of the 2D games, followed by the first 3D iterations of those games (Mario 64, Ocarina, etc...) and the N64 rare games (Goldeneye, Banjo-Kazooie) and then the popular appeal of the Wii. What defines the Gamecube? The weird controller? The minor hits? Good iterations of Nintendo franchises like Mario Kart and Zelda (All Nintendo systems have good iterations of those franchises.)
Once again, it's not a bad system at all. I had one. I like Super Mario Sunshine. I liked a lot of games for that system. But it was third fiddle during its lifespan and it lacks megahits. It's just an understated pleasant little system. And that's why it doesn't get attention.
AND THE DISCS ARE TINY!