The idea of Poles as a boring and mentally challenged people is a fairly recent and possibly American invention.
The Poles have a long and rich history, both intellectual and cultural. Copernicus was Polish. So too was Frederic Chopin, the embodiment of romanticism in music, and one of the greats in the pantheon -- a brilliant pianist and womanizer, who tragically (and fittingly) died young.
The trope of the dashing young Polish cavalry officer was once a literary staple. Until fairly recently, the idea of a Polish joke would have simply made no sense at all.
To be fair, Poland has had more science/math come from it than culture.
Marie Curie (nee Maria Skłodowska). Hilary Koprowski invented/discovered the vaccine for Polio. Mandelbrot, self-explanatory. Magnuski invented the walkie-talkie. Zurek, one of the current leaders in quantum theory, Polish.
Polish art/literature doesn't have nearly the same exposure as Marie Curie and Mandelbrot. Someone who doesn't care to look into it wouldn't really know.
(Though to be fair, this could be my background as an engineer shining through. These are the people we learn about. And Pope John Paul II was Polish, too.)
(ETA To clarify: I'm not saying Poland hasn't had amazing art. I'm talking about what the common westerner -- and really I should say American -- might have heard in the course of their lifetime.)