This is of course a stereotype because people of TriCity are perceived as the most laid out Poles all over the country.
and by laid out you mean laid back...
that's the beauty of stereotypes, they don't apply to everyone, they don't even have to apply to most, and everyone is free to hold on to their own.
frankly, I don't see the Poles as uptight, per se. And I know many more Poles than just my husband, to dispel the assumptions. I think Poles are really fun, just very insecure and extremely given to self-defensiveness. I believe it comes from the history of the country, when the Polish people found themselves on the defensive much more frequently than on the offensive, and had to obsessively defend their language, culture, and traditions, even if it meant going "underground." Xenophobia is a self-defense mechanism. Also the fact that they see other nationalities that are more self-assured as naive and uncultured. I also see that the acceptable standards of politeness are lower than in some other countries, but perhaps as more people travel, they'll notice that being friendly does not mean compromising your position.