Polish jokes exist because there were vast numbers of Polish immigrants who arrived here in the late 19th and early 20th centuries. Most of them spoke not a word of English, and stupid, ignorant nativists assumed this was because the Poles were stupid or ignorant.
Polish jokes are practically unknown in Europe, and a source of mystery to Europeans who associate Poland with Chopin, Madame Curie, gutsy Lech Walensa, lovely old cities like Krakow, and brave, doomed cavalry charges against the Nazi tanks.
Even in this country you don't hear Polish jokes all that often any more. My school-aged kids have never heard one.
-ccm