"Polish jokes" became a fad in the Chicago-Milwaukee area around 1965.

They were so common that there was an "Official Polish Joke Book" published in 1966.

Whatever the ultimate source (Germans have been blamed), the American "Era of the Polish Joke," when everyone seemed to be telling them, ended (in my experience) about ten years later.

They're still heard but not very often. There are probably some "tradition bearers" who know dozens of them and will tell them at the drop of a hat.

Before the "Polish joke" fad was the "elephant joke" fad of about 1962. There was also a fad for "dead baby jokes" around 1970.

The people I knew who told Polish (or, as they were usually called outside of the media, "Polack") jokes seemed to have absolutely nothing against real Poles. It was just a rhetorical convention.

Which isn't to say that real Poles weren't justifiably irritated.

Fun facts: Though "Polack" is now derogatory in English, it's just the English pronunciation of the ordinary Polish word. It's even in dignified use in Hamlet, whose father "smote the sledded Polacks on the ice."