It’s possible that the “blondes are dumb” stereotype arose first from random noise and then was reinforced in other ways. For example, I have seen blondes who were actually not very intelligent deliberately acting as stupid as they could. They were purposely playing to the stereotype and apparently believed that it looked cute. But other people who did not notice that they were doing it deliberately might have had the stereotype reinforced by that behavior.
It’s possible that the “blondes are dumb” stereotype arose first from random noise and then was reinforced in other ways. For example, I have seen blondes who were actually not very intelligent deliberately acting as stupid as they could. They were purposely playing to the stereotype and apparently believed that it looked cute. But other people who did not notice that they were doing it deliberately might have had the stereotype reinforced by that behavior.