I think like some silkies, a lot of the dumb Ness has to do with if they can see well, though to be fair my male silkie has hardly any crest and he's dumber than an empty skull.
But I had a buff laced polish with a giant, beautiful crest that was very.... not smart. That also could have been the result of a damaging peck to the back of her he'd when she was a pullet that made her almost comatose for a few days.
The black crested whites I had had much smaller crests and could see relatively well. One learned that if she wanted to be carried, she had to climb the stairs to the second level deck and peck at the window