Actually it is freakshows who REFUSE to wear a mask and create a scene screaming about their "medical exemption" and other false nonsense who qualify as Karens too.
But it's also being used by black people as a derogatory term for a white woman and particularly one they perceive as exhibiting these crazy and entitled behaviors. From what I have seen on twitter, some black women like to consider every white woman a karen no matter what (which is racist, obviously).
There are tons of legal medical exemptions for masks in many countries for severe respiratory aliments.
As we can see from Kahane was right BT, Karen was something that had a loose definition, and now everyone is using it to mean what they want. The left tried to say the people defending their homes from rapey looters are karens, or anyone who calls the cops for anything.
The two types of memes that focused on karen were "can I speak to the manager" and "bring back the f-ing kids, Karen". A self-entitled bossy pants who is happy to try to ruin everyone's life and day on a whim. There was that lady who called the cops and said a black guy is trying to kill me when he was being a karen and trying to give treats to her dog so she would put on a leash. The mascot of Karens is the middle aged woman with a short blonde haircut that is always upset about something.
For covid, you have anti-corona karens that go into stores and refuse to leave when asked because the store likes masks, and starts screaming about their rights, and you have the corona karens that scream they want to measure the distance between people on the beach and scream at you because you didn't put your mask on in the one second you stood up on your seat for example.
It's people that don't care who they step on to satisfy small emotional anguish, usually expressed by harassing managers and taking the kids. Besides the unclear definition of what that is and the fact that internet terms are always abused, many karens don't know that they're karens and call people who call them out karen. Even if you go to the wiki definition, "anti-vaccination beliefs, being racist", those things don't necessarily make you a Karen. The left is using it as a weapon against people they don't like, but being a Karen involves actions and not thought crime.