What's so cold about this joke? blonde

There is prejudice against blondes, and it is generally believed that blondes have no brains (this shop assistant is such a person). The blonde said that she would buy a TV, but in fact it was a microwave oven. In other words, although she dyed her hair black, the stupidity of the blonde can't be changed. Anyway, stupidity is a sign of blondes, and people can know it at once. Personally, I think this view is wrong. )

I think this is the meaning of this passage.