According to Edge and EEDAR's numbers, 17 percent of PS3 games have scored 85 or higher, while 30 percent are rated below 65. This compares favorably with the Xbox 360, which saw 13 percent of its titles above 85. The Wii sees only 4 percent of its titles rated 85 or higher, while a full 54 percent fell at or below 65.
Those who follow the industry can probably guess why Wii seems to attract a disproportionately large number of poorly reviewed games. Edge cites the low entry barrier for Wii developers, allowing more bad games to hit the market hoping to ride the Wii cash wave -- or, if you prefer not to mince words: shovelware. The report also points to developer difficulty in grasping the intricacies of Wii's control interface. Furthermore, the article posits that review scores don't really matter to the system's primarily casual audience.
Of course, the critical caveat concerning the analysis is that review scores are subjective. After all, one gamer's must-have is another's don't-bother. Just look at Fatal Frame IV.
Source - Games At-a-Glance: PS3 Quality
Source - Games At-a-Glance: Wii Quality