First, I agree that bigotry is unacceptable on our sites. We have a very firm policy against rudeness.
Second, I edited the question to be clear upfront that the stereotype is demonstrably false. Polish people are as likely as anyone else to be intelligent or not. I hope that addresses your specific concern. (If not, I'd be happy to learn more.)
Third, I wanted to address the broader question of the study of history that you implicitly raised:
What offends is the hidden presumption in the question that the stereotype is rational, rather than an emotional outburst of bigotry.
Reading between the lines, it seems to me that asking for the reason behind something that happened in history is where the presumption is hidden. For many topics, that's a fair reading: if I can identify the reasoning for concluding that pi is irrational, it's rational for me to believe that mathematical fact.
But historical reasoning is often different. Knowing why Lieutenant Henry S. Farley fired upon Fort Sumter does not make that action rational. History abounds with bad reasoning resulting in monumentally bad ideas. Asking where such ideas originated does not validate them as ideas. A considerable portion of the study of history, in fact, boils down to the question: "What were they thinking?!?"
Finally, we have seen questions that assume or imply racism on. Fact is, bigotry of all types exists. The best place to dispute racist assumptions is in an answer. Next best is a sympathetic edit. (Sympathetic is key here. Sometimes people don't know or understand what the problem is. An edit that allows a question to be asked without offending folks can help an asker save face.) Third best is raising the question here, in meta.
I appreciate your thoughtful update to the question. The first version was perhaps too abrupt and sparse to be productive. It was helpful for you to provide your analysis of the problem.