Have we decided, as a culture, that it's acceptable to take the piss out of women who just happen to be blonde? Or do we really think that blondes are stupider? Can you imagine this joke being told about blond men? Note the classic strategem of apologising to the group maligned in the joke, too. ("Oh no, flick or hhfishlifter. I didn't mean you! Obviously you're not dumb. But everyone knows that 'blondes' in this joke means 'generic group of people mocked for their stupidity'.")
Both men and women shown pictures of blonde women judge them to be stupider than pictures of the same women with dark hair, red hair or grey hair. So I think it's a pretty safe bet that they're discriminated against at school and in the workplace too.
Blonde women tell tales of being systematically put down all through school because of their hair colour. And there are far more blonde jokes now than there used to be. Now, it's fine for adults to dye their hair whatever colour they like, but we don't normally encourage children to dye their hair. Is it reasonable to expect blonde girls to put up with people assuming they're stupid, and making jokes at the expense of their ethnic group?