It's "Karen" in normal conversation, but if she brought storebought cookies to the potluck it's "Kar-in-nuh", with the emphasis on the last syllanble.

In my head, these are called "spite syllables" :)

That would usually be coupled with a very veiled reference to how she put in the minimum possible effort while indirectly insulting her entire bloodline: "Oh, don't you just feel for Karen? I don't know how she gets anything done with those kids of hers."

Alternatively, if the speaker wants to embarrass her in front of someone with a higher social standing; "Karen, these cookies are just the best thing ever! What do you put in these to make them so good?"

Hah. Where I live this would mean they have marijuana in them.