Blonde Vs all men, is just like any other jokes, where a particular group is laughed at.In India it could be the Sardarji Jokes, In UK it could be the Scotts and even in the arab world, this joking of a partiular country is prevalent. Recent example was US bias against the French as they opposed the Iraqi war and subsequent "French Jokes" that were passed around. In almost every country there are such biased jokes- especially when more than one culture/lifestyle cohabit. Example are red Indians in USA or even as in unifed Germany having bias to Easterners. But blonde jokes are more of male sexist jokes.
Maybe its because blondes are more attractive than brunettes and blondes realising this may give their beauty more thought than anything else that goes arond them!