Cleve Evans, a professor who studies the history of names at Bellevue University, told writer Aja Romano at Vox that names have been used in this way for ages. "They are just another linguistic item that's possible to be associated with a particular group because of the perception that it's common among that group."
Evans also pointed out that the name "Karen" is a Danish form of "Katherine." That name derives from the Greek "Aikaterine," which is sometimes confused with the Greek word for "purity." People using Karen on social media probably didn't dig deeply enough to make that particularly ironic mistake.
Before Karen, there was Becky, whose cluelessness reigned from the 1990s into the 2000s, from Sir Mix-A-Lot's "Baby Got Back" to Beyonce's "Sorry." In April 2018, "BBQ Becky" called the police because Black people were using a charcoal grill in a park. But the modern Karen is more than clueless. As NPR's Karen Grigsby Bates put it on NPR's Code Switch, she is "aware there will be consequences if she summons help — and those consequences will fall most harshly on Black people, usually Black men."
Cases like these have changed the Karen stereotype from being someone who demands to see the manager to someone who calls the cops. Having harsh blond highlights is now the least of Karen's many issues.