I could believe the Wii has a higher proportion of poor titles, and I do not believe it is reviewer bias. The reality is that the Wii started with a bad stigma created by the previous two consoles, and then the stigma did an about face into another bad direction. The console started with an abundance of preteen garbage. Nobody accused eight year olds of having fantastic taste or a critical eye towards quality. They buy the game, because it has their favorite cartoon character on the front. Then third parties jumped on to the casual mini game market, or worse very dated ports.
The console just doesn't have a fantastic lineup. The carry over karma hurt the launch lineup. The success of Mario Party 8, Smooth Moves, Carnival Games, Wii Play, and Raving Rabbids then created a poor perception. The console got a plethora of poorly made yet highly selling party games. When developers saw that quality was of no real concern they gave the console the mark of Cain as it were. Instead of labeling the console kiddie this generation they have instead labeled it the party game machine.
The reviewers appear to have been critical, but that is their job. I love to read the attacks on Wii Sports reviews. The reality is if you judge the game as a stand alone title it just isn't very good. Sure you can think highly of it, because it was free with the console. However would you pass up a Mario Galaxy for Wii Sports, or even a Resident Evil 4. Would Wii sports be worth fifty dollars?