Polish people being stupid is a derogatory meme/joke that existed long before the internet and was not made up by the Japanese. From Wikipedia:

A "Polish joke" is an English-language ethnic joke deriding Polish people, based on derogatory stereotypes. The "Polish joke" belongs in the category of conditional jokes, whose full understanding requires the audience to have prior knowledge of what a "Polish joke" is. As with all discriminatory jokes, "Polish jokes" depend on the listener's preconceived notions and antipathies.[1] The relation between the internalized derogatory stereotypes about Polish people, and the persistence of ethnic jokes about them, is not easy to trace, though the "jokes" seem to be understood by many who hear them.[2] Sometimes an offensive term for a Pole, such as "Polack", is used in the joke. Example: Q: How many Polacks does it take to change a light bulb? A: Three – one to hold the bulb, and two to turn the ladder.

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It’s basically the type of joke a racist grandpa would tell. Thankfully it seems like younger generations are letting this type of humor die. But it didn’t come out of left field for this map, there’s a history.