I will second @strim.
Not because I'm Polish, but because it's not true.
@slaapliedje,
In general it's not nice to judge whole nation based on one example. Even if this example is 100% real.
In every nations they are good and bad people, hopefully the first type is majority everywhere.
Jokes are subject to discuss, but I can tell you that we have a lot of jokes about Americans too. Hopefully you don't know them. ;-)