I was told that the reason the "dumb" Polish jokes stuck is because the Polish were the last group to immigrate to this country back during the big influxes from Europe.
When the immigrants first land they are very unfamiliar with the ways of the land and of course not completely familiar with the language.
So as a group they appear a bit dumb about lots of things.
Before the Polish there were other groups, Italians, Irish, German, French.
As those groups landed, they were the brunt of the dumb jokes, because when they each first landed they appeared to be dumb about the ways of the land.
Apparently the Polish were the last group of those big waves of immigrants and so the dumb jokes got stuck with them.