I think the Karen label is along the lines of "a woman perceived as entitled or demanding beyond the scope of what is appropriate or necessary "
I had never heard the the term till late last year when my youngest daughter who is studying Psychology at Uni started using it and still uses it till this day. She has had a number of customer oriented roles while she is undertaking her degree, starting in cafe and then moving into retail, and then after the pandemic started and retail closed into CSR call centre work (mainly from home).
Off and on and mainly as it just gives my daughter the chance to vent at the end of her working day she will talk about various issues of her day. Sometimes positive about nice interchanges, and sometimes negative. My daughter states that Karen's have been the bain of her working life and the only group, or even stereotype, that she dreads. And it is not just the "entitled or demanding beyond the scope of what is appropriate or necessary" it is the vitriol and abuse that occurs and which she only rarely experiences from others. She certainly gets some other angry customers from time to time, but they in her words seem more motivated by the issue, whereas the Karen's seem more to just want to exercise power.