I think games are receiving average graphics scores because there are issues with them that relates to more than horsepower. Zelda, for example, is mainly receiving criticism for pixelation and poor textures, which is because it's a GCN game. But it's not a GCN game, it's on the Wii. And in a review you can't give a game a great score just because it started on another system. I'm sure if just GCN it would be receiving 10s in the graphics. As is, we KNOW Nintendo purposely chose not to improve them for the Wii. They are admittedly not top-calibur as far as the Wii is capable of (even for first gen games). There's nothing wrong with that, but when rating that specific aspect it's only fair.
Let's be honest, something like SMB looks great and far superior to its GCN version. But does it look better in relation to the original when it debued with the GCN? Let's compare the graphics scores of the original SMB and the SMB:BB.
SMB
7.0: A superb, constant fluidity of 60 frames per second is a relief. But otherwise nothing terribly special.
SMB:BB
7.0: Crisp, clean, colorful cel-shaded visuals are a natural fit for the franchise. Runs at 60 frames in pro-scan and 16:9 widescreen. Still, the graphics are very simple.
In otherwords, both are nice but rather simplistic. Not a bad score by anymeans, but it leaves plenty of room for superior games. And, IGN gave BB a better overall score than SMB, so there you go.
To make another comparison, both Wave Race: BS and ExciteTruck received 8.0 for their graphics (WR did do much better overall, though). Both reviews like the graphics but found them to be similar across levels and nothing particularly special or memorable. Sounds like a good 8.0 to me.
And, oddly enough, Tony Hawk earned 7.0 for both its first GCN game and its first Wii game.
These graphics scores are in no way abnormally low...