Assuming he gets the nomination, he has to go with somebody with political experience and stature in the party. Carly’s an amateur and makes Cruz look amateurish, and would anybody on the fence really decide to vote for Ted because Carly’s on his ticket?
If somebody takes McCain's comment to mean that Carly is warm and sweet and will make Ted Cruz all warm and fuzzy it's wrong, but Ted has so many sharp corners that even somebody like Carly could round them off a little. I suspect it's a bit more of an after-the-fact rationalization than anything else, but Ted's in an almost desperate spot now and I can't blame him for grasping at whatever hope he can come up with.