Dunno, where I live, it's used a lot. Of course there are a lot of other words, and I cannot say if "Möpse" is the most commonly used term, but it is at the very least in the top five I'd say.
I assume it is because those are only some of the many names and even if those would really be the most used, it is still only a small fraction overall.
i didn't downvote but imo it is light on data. having some more info on methodology would help, like where did the data come from? was there any normalization or other processes?
It's hard to know if this is a map of words used most in an area relative to how little it's used elsewhere, or just most common? or just..cherry picking slang that isn't actually commonly used but is from that area?