Um, it's worth pointing out that Madworld isn't typical Wii fare - it's more of a 'hardcore gamer's game' and the Wii is very consciously aimed toward the 'casual' gamer. So the fact that "only 8% of the Wii market had even heard of MadWorld" may be more of an indication that only 8% of the Wii market is actually hardcore gamers. Madworld was only ever going to appeal to a subset of Wii owners - it looks like they may have overestimated the size of that subset.
For the same reason, these results don't indicate that "Metacritic scores don’t hold as much weight with Joe Gamer". "Joe Gamer" - on the Wii - doesn't go to Metacritic. "Joe Gamer" for other platforms, though, may well put more weight behind game reviews. They are different "Joe Gamers".
If we're going to use movie analogies, the family market for movies (PG-13 and below) is significantly larger than the market for R movies. But, to continue the analogy, the fans of horror movies would probably be a lot more swayed by a good review from Fangoria magazine than "Joe Moviegoer" in general would be.
The take-home lesson here is "know your market", not "reviews are unimportant".