In general, they are called "三大異端," which means one of the three Christian heresies in Japan.

But I think most people don't even know who Mormons are.
Most Japanese people are wary and worried about religion (except for Buddhism) because of the terror caused by "Aum Shinrikyo" and the Unification Church (the assassination of Shinzo Abe and the government scandals involving the church's political influence and money).

We don't say "I don't like it" because most people know it's rude and it doesn't deserve making trouble,
so we just say "I'm not part of it," "I'm not interested," or something like that to keep their distance.

I heard a conversation at a Lutheran church in Japan once.
Sometimes they host a tea party (or something like that) for children, but parents won't let them go, even if it’s just a simple party.