Taken by itself, obviously it is meaningless for evaluating a football prospect. Moreover, it is really only useful at all for gauging the value of a prospect, not a professional with an existing body of work. Brett Favre's wonderlic score is completely irrelevant because we no longer need to predict his aptitude.
To a lesser degree, the same is true for Campbell. Citing his wonderlic score at this point is very, very peculiar. The OP references the last seven Superbowl winners, but why stop there? Why such a small sample? QBs tend to have much higher wonderlic scores than other position players, so it's not surprising that a small sample of elite QBs have high wonderlic scores, but obviously it's idiotic to suggest Favre and McNabb are incapable of leading teams to SB wins because their wonderlic scores are not high enough. Likewise, Payton Manning is not an outlier for winning a SB with a sub-30 wonderlic.
So, basically, I don't understand this thread. Are elite QBs smart? Typically, yes! They must be able to read differences, understand coverages, make quick assessments, and have a grasp of the playbook. But is Jason Campbell somehow less likely to win a super bowl than, say, Alex Smith because he has a lower wonderlic score? Of course not. Like I said, body of work is the best way to evaluate a QB's relative effectiveness, not measurables.