Even if the graphics in MK Wii could've been better, in reality the series was never about pushing polygons.
The first one had impressive Mode-7 effects and the sprites were colorful, but was pretty basic looking, especially when compared to the other SNES games.
Mario Kart 64 featured washed out, low-res sprites against basic and polygonal backgrounds.
Mario Kart Super Circuit looked good, but the colors were a little too pastel and the sprites were small and pixelated.
Double Dash!! was perhaps the best looking game at the time mainly because there was so much going on in the background, the vibrancy of the color and the smooth frame rate that it was hard to label this as inferior looking. The karts and character models, though, were low poly, especially when the MARIO PARTY series on the GC boasted better models.
MK DS looked impressive, mainly because it was running on a small handheld and thus the amount of detail along with the speed made it a looker.
Finally, MK Wii looks basic in character modeling. However, I think the tracks are beautiful thanks to both boasting great and even fun details like statues and Miis to some effects like bloom and such.
So overall, complaining about the graphics is silly to an extend since from the very beginning the series was never meant to push graphics, but multiplayer mayhem.