The dumb blonde cliché started with the kinds of characters Marylin Monroe played in many of her movies. She usually played characters who were pretty ditsy, though sometimes she came through with some profoundly wise insight. So when people blame their ditsiness or naivety on being blonde, they are just pandering to a popular culture stereotype.
In other words, being blonde has nothing to do with being stupid. Being stupid has everything to do with being stupid.
Don't over analyze it.
Dude:Don't be a fucking idiot.
Idiot: But thats the highest rated definition for 'blondes' on urban dictionary dot com!
Dude:Well, whoever defined it like that is an ass. Besides, they defined it as 'long' dicks you fucking dickless moron.
A common trend in modern times is for people to dye their hair blond using a certain hair coloring agent. This trend is due to the depictions of blondes as symbols of beauty and attraction in the art and media outlets.