It's kind of funny to use the phrase "dumb-looking," then employ a scientific study to back up the rest of the story, but that's just what happens here.
In a study at UT-Austin, men were rounded up and surveyed, using scientific methods that are laid out in this article ad nauseum, about what they look for in women, and what attributes they find most attractive.
It turns out that, in a nod to an almost prehistoric outlook, that the men found "dumbness" or at least the appearance of dumbness to be a turn on. Same goes for drunkeness by women.
Of course, there's a lot more to it than that. The drunk and dumb ones also tend to offer more in the way of sexually suggestive behavior, but even when those factors were isolated (again, using complex scientific methods that would make my eyes bleed) the dumb ones of the bunch still came out on top.
It sounds misogynistic, but there's an innate part of the male psyche that (supposedly) looks to score at any opportunity, and no amount of 'Grey's Anatomy' will shut that off.
Ok, maybe a TON of 'Grey's Anatomy' will, but that's the precise reason why we shouldn't be watching 'Grey's Anatomy' to begin with.