The reason for Polish jokes is really very simple. First, Poland. has few natural
geographic barriers which make it difficult to defend, and those it has are far behind the Polish border. Poland is located on the Northern European plain between
Russia and
Germany, which places it on the most natural
invasion route between
Western Europe and the east. Caught between ambitious powers with little geographic protection Poland is fairly easily, and often, conquered.
Second, until the Holocaust Poland had the highest Jewish population in Europe. The combination of anti-semitism and an almost indefensible, and thus oft occupied, country made the Poles a natural butt for jokes. Add the usual cultural differences of new immigrants and you have reality enough to support these jokes.
That said, a number of the Polish Americans take those jokes philosophically and often tell them themselves under the theory that the best defense is a good offense.