Are Polish jokes funny because you know a dumb Pole? Unlikely.
I don't think the humor in these jokes has anything to do with truth - they could apply to any group with a reputation for being dumb.
What's interesting is they probably would not be funny if Pole or blonde were just replaced with "idiot," probably because the listener doesn't get to make the logical leap to stupidity themselves. There may also be an element of shame and schadenfreude for picking on a group that adds to the humor.
I don't think the humor in these jokes has anything to do with truth - they could apply to any group with a reputation for being dumb.
What's interesting is they probably would not be funny if Pole or blonde were just replaced with "idiot," probably because the listener doesn't get to make the logical leap to stupidity themselves. There may also be an element of shame and schadenfreude for picking on a group that adds to the humor.