Both, but mostly the latter. Getting a useless troutlord on the Throne tends to spark rebellions anyway though, and a bad King means more potential for those under him to do stuff even if it doesn't devolve into civil war.

Also, because Targs always have claims on Westeros, it's possible that a really bad monarch would cause a faction demanding a Targ be installed as monarch instead to pop up. I'm not banking on this by any means, but if it happened it'd be pretty nice.