You might want to eat those words now. Most of the Wii launch software was either horribly rushed (Far Cry), didn't need "realistic" graphics (Trauma Center), was a tech demo (Wii Sports), or was originally a GameCube game (Zelda). There was also a great deal of licensed shovelware that would not have been programmed to ultimate graphical spectacle levels anyway. Publishers wanted to get as much product out as possible to capitalize on the predicted success of the system (anyone who thought the Wii wasn't going to be an enormous success needs to get out more). Unless you can see into the future, the above claim is totally ridiculous.
In the Wii's defense, ExciteTruck looks at least like an XBox game, and Zelda is one of the best looking GameCube games. One was hastily programmed, the other was for another system. If Doom 3 or Chronicles of Riddick were ported to the Wii with absolute care, from scratch, they would look as good as or better than the XBox versions. I think the benefit would be seen most in frame rate.
A majority of multiplatform GC games looked just a hair below parity with their XBox counterparts. The lack of online play was the real difference. Resident Evil 4 may be the best looking game of the last generation, and it was a GameCube game. On the flip side, God of War was a spectacular looking game, on inferior hardware. It all depends on the developer.
The Wii will continue to do well because, whether you'd like to admit it or not, the wand works and really has changed gaming in a way on par with the move from 2D to 3D. It will command a large marketshare, and in turn, developers will pour more resources into developing for it. The Wii will never touch the a/v capabilites of the competition, but its a sure bet that there are better visuals coming in the future.