The name “Mormons” is a nickname deriving from the Book of Mormon. It has never been the official name of our Church, which is, the Church of Jesus Christ of Latter-day Saints. Groups such as the RLDS and the polygamist breakaways are no longer “Mormons” because they are no longer members of the Church of Jesus Christ of Latter-day Saints. Some still share some of our doctrines, I think. But since priesthood authority is the hallmark of the restored church, and since they reject that authority, I would not call them “Mormons.”
The “Reorganized” church, BTW, is no longer called that. They call themselves the Community of Christ. They seem to share our doctrines less and less, from what I can determine. They joined the World Council of Churches, for example, which I’m told means they have to accept the Nicene Creed.
Since the LDS reject the hallmark of the Trinity, some would not call them Christian.