I know mainstream Christians, in a rare display of an ability to recognize inanity, like to pretend like Mormons aren’t also Christians, but let’s illuminate the differences between these sects with a conversation:
Mainstream Christians: It’s just silly that Mormons believe Joseph Smith received gold tablets from an angel on a hill.
Mainstream Christians: Everyone knows Moses received stone tablets from a burning bush on a mountain, though.
Atheists: I do believe we have parted ways, my friend. My crazy, crazy friend.
I can’t say I see any importance distinctions.