I'm not sure how this is an overreaction on her part. Worthless. He used the word worthless. Joking, yeah not so much. He'd be lucky he made it out the door with his head intact if it were me.
Yeah, even if he was "joking" ... that's a shitty thing to say to your spouse. I'm really not sure why so many people think this kind of stuff is okay or should be laughed off. It sounds like there's more to the story, too, and this is the kick that prompted her to say something.
That word just makes my skin crawl. Even with no history, no backstory, to me, that is a fucked up thing to say to someone. Say that to your worst enemy, maybe, but never ever ever your spouse.