The speaker is saying more than that. The speaker is probably saying that he/she does not believe what the other person is saying. They are not just bad at lying, but they also likely just got caught in a lie. This is the speakers way of letting them know, just before he/she gives proof of the lie.

yes, but it is very informal, something a child might say.

You are very bad at lying - not necessarily a bad thing.

"You stink at lying." = You can't lie for nuts [ Aussie slang] or you can't lie to save yourself

Yes, it means you are bad at lying. This is something that friends or couples usually say to each other.