The thing you have to note is that the programmers are just getting use to the new Wii system. As time goes by programmers will get used to the system and discover the Wii's hidden talents.
Here's a comparesion:
When Burnout 1 came out for the PS2 it look pretty bad.
Burnout Revenge:Now after five years of developing for the PS2, programmers now have better exprience on programming for the PS2 and the graphics look a lot nicer.
So it's normal to have crapy graphic at console launch, but as time go by the graphic will improve.