My wife says she is going to kill my daughter (her step daughter) out of frustration.

My wife says she is going to kill my daughter (her step daughter) out of frustration. I know she wouldn't really hurt my daughter, but I just think saying you are going to kill a child out of frustration is a bit harsh.
To my knowledge, she has never said this directly to my daughter, just to me via text.
When it happens, I tell her that she's being overly dramatic, or that I do not like it when she says she will kill her.

She doesn't see it as a big deal and says she is "joking", and it starts an argument. She claims that I am too critical of her.

Am I being too critical?