The Karen meme. Is it funny? Is it sexist? Don't ask me

You've likely come across a Karen, or at least the Karen meme (or trope, depending on your choice of terminology).
Generally speaking, a Karen is a stereotype of an over-achieving middle-class white mother of multiple kids, the kind who tends to complain about things. She may well be an anti-vaxxer, is probably divorced, likely sports a bobbed hair-do, is addicted to Facebook, and sees complaining to the manager as her purpose in life. There are those who contend that it is sexist, racist, ageist, classist and probably several other -ists as well. It is certainly pejorative, disparaging and condescending. But, as a shorthand for a particular segment of society, it has been extremely successful, much more successful than similar attempts to brand Becky, Susan and Tammy.
But how did all this start? Well, generally speaking, social media, but there are several possible origin stories, including a popular subreddit, /r/FuckYouKaren, which started as one guy's diatribe against his ex-wife in 2017 before exploding exponentially in 2018 (we are probably not supposed to use the phrase "went viral" these days). Or possibly, going even further back, to the 2004 movie Mean Girls ("Oh my God, Karen, you can't just ask someone why they're white"). But it's almost part of the point of these social media memes that their origins are murky and/or disputed.
Personally, like much of Millennial humour, I don't find Karen memes amusing (try these, supposedly 27 of the funniest Karen memes, for example). And why the need to attach a particular name to them, I have no idea. Does that make them funnier? Ask a Millennial.