I'm thinking of it from the perspective that, if my name is Cathy, and someone else is called a Chatty Cathy, that wouldn't necessarily feel like it applies to me because I might not consider myself to be chatty. Ie, Chatty Cathy is a subset of Cathies, not all Cathies (or a subset of people, not all people). But as there is no explicit modifier used with Karen, then if my name is Karen that usage automatically includes me in the group. But I agree that both usages are bad and should be avoided.

In general, I don't think that insulting people is good or useful anyway, and that calling out / discussing the objectionable actions is more useful.