Mormons do get drawn outside of Christianity by some christians for a variety of reasons (non-trinitarian, open canon, cosmological background quasi-polytheism), and some people have suggested classifying the faith as another Abrahamic one as a means of sidestepping that discussion, which may be where one could get the idea that the comparison to Muslims is a good one. But if Mormons are outside mainstream Christianity, it's only marginally so.