I remember in a previous thread that I proposed this definition:
A Christian is a person who believes that Jesus is the son of God, is a follower of teachings ascribed to him, and does not believe in god(s) other than the god(s) of the New and Old Testament.
Parenthesis there are to allow Gnostics or Christians with “heretical” views on the trinity to count, while excluding, say, a polytheist who adds Jesus to a pantheon.
Now I'm not an expert in Mormon theology by any means, but I believe that the Mormon concepts of the Heavenly Mother and of humans becoming gods (godlike? this is where I'm unclear) in Heaven would fail this definition I made up. It's an edge case either way.