"Christian" originated as a demeaning term, intended to mock people who followed Jesus or Jesus' teachings. It'd be the equivalent of me calling someone who posts a bunch of pictures of their butt on the Internet a "little Kardashian". Christians who were first given the title saw it as a good thing, of course, since the meaning was basically someone attempting to act like Jesus. Mormons are pretty open when it comes to taking about their faith, so if you don't understand why they use that title, maybe you should just ask them in what way they try to emulate Christ. I'm sure they'd answer that one for you.