Yes, blond girls can stupid, but brunette, and ginger girls can also be stupid. It's a stereotype founded by the idea that an attractive woman could get through life by her looks -examples such as playboy, where in the early days they were mostly blond - which lead to the generalisation of blond girls. I know blondes that are stupid, I know blondes that are smart, the same goes for brunettes and gingers, it goes both ways (like your mum :3)
Although, all stereotypes have some truth, for instance, the English stereotype is that they play cricket over a nice snack of tea and crumpets, this is partly true but the majority of England would take the cricket bat and smack you round the head with it, east London style!
there is no way that hair color determines level of intellect but to be honest a lot of girls really do drive the clique home and prove it to be true...most times those girls aren't even naturally blond, I know a couple of girls who are brunettes who are about as smart as a rock. Honestly though when I see a blond girl I don't immediately think that she's smart...its a terrible stereotype.
Girls who go blond tend to purposely live up to the stereotype.
You even referred to your dumb moments as "blonde moments" and when you say that,it only adds to the stereotype and makes me think you probably do try to play dumb.I'm not saying you,are but your choice of words makes me think so.