Why does blondes are always called dumb?

Because the stereotypical blonde is pictured as "attractive" at the expense of "intelligent". In the West, at least, attractive women are said to have less impetus to develop their minds.

• ALL blondes are NOT dumb. But some have gray matter that is partially bleached out.
• Blondes aren't dumb. Non-blondes are just jealous, so they call them dumb. It is a myth. After all, "blonds" are generally acknowledged as brainy.

So everyone else (brunettes, readheads, black-haired and grey-haired women) are always and exclusively intelligent? Wow! That is AWESOME! - I think I will buy myself a dark haired wig.

Or, could it be that dumb is dumb, as nasty is nasty, as nice is nice, no matter what the hair color?