I never saw Nords as dumb, just stereotyped as such by other races and sometimes rather headstrong. Besides, they are very charismatic and have a great deal of willpower, so that stands for something.
Charisma, intelligence and willpower are different matters. One doesn't need to be smart to have a strong will, and there may be people who are good at getting others to see things there way while at everything else, they act like idiots. I'd say that, as a whole, we can likely assume Nords are stupid, because they're a race designed with warriors in mind, and in fantasy RPGs, this usually means they're not going to be very good at being mages or thieves. It may not be logic that is supported by the lore, but the Elder Scrolls series is a video game first, the lore was just added on to enrich it, so rather than designing gameplay based around existing lore, one can expect that a lot of the lore is written with the gameplay in mind.
Right, but in MW argonian players have low personality scores. But who is the guy with the highest speechcraft (and I assume highest personality therefore)? Skink, an argonian.
That's because the low personality represents the race as a whole, whereas individuals may be exceptions, the same goes for Nordic stupidity or any other general racial stats in the game.