If it's truly a mistaken perception on the part of observers, that mistake is unlikely to be widespread enough to generate a stereotype with absolutely no basis in reality.
There are probably exceptions to this, like the "blondes are stupid" thing, because intelligence obviously has no correlation with hair color. So, you have a point that this might happen sometimes, but most stereotypes are based on some small grain of truth.
But the fact that it's true for people from that demographic more often than it is for people from any comparable demographic means there's a grain of truth there. It's still only a grain because it's probably not true of a majority of people in that demographic--but enough people from that demographic do it to create the impression of a well-known stereotype.