The simple biblical definition of what is a Christian can be found in Matthew 16:15-17
15 He said to them, “But who do you say that I am?” 16 Simon Peter replied, “You are the Christ, the Son of the living God.” 17 And Jesus answered him, “Blessed are you, Simon Bar-jonah! For flesh and blood has not revealed this to you, but my Father who is in heaven.
While Evangelicals would like to add all sorts of additional requirements to those given by Christ himself, by this straightforward biblical definition, Mormons are most certainly Christians.
This definition, by the way, also has the additional benefit of distinguishing Muslims from Christians; Muslims accept Jesus as the Christ/Messiah, but reject the idea that he is the Son of God.