From my understanding a "Karen" is a woman who complains about minor things, especially in a restaurant or retail setting. I think this applies to pretty much every woman I have ever known (and a fair amount of men). With my current wife, her acceptance rate on the first table we are shown at a restaurant is about 65%, so every third time we go to sit down, she asks to sit somewhere else. Similar thing (though not as common) for hotel rooms.
I think that makes her a "Karen", although I am not near dumb enough to tell her that
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