I wouldn't personally call it racist or sexist (as I've seen the term used for both men and women and of different races). However the logic works like this:

Tyrone is a internet stereotype of a thuggish blackman. The stereotype of Tyrone is considered by some to be racist (probably quite rightly). Karen is a stereotypical entitled white woman. So if calling someone a Tyrone is racist, therefore calling a woman a Karen is also racist.

Not saying I agree with any of it. But that is my understanding of how the logic works.