Christianity as a whole began as a cult of a cult of Judaism and so Mormonism began as a cult of Christianity, and all of it is several cults removed from Zoroastrianism anyway, with a bit of Greek and Roman paganism for flavor. Arguing about which is "more legitimate" is as ridiculous as debating how many angels can dance on the head of a pin. A thousand years from now, assuming humans are still around, they'll probably be worshiping the Omnissiah or something.

It surprises me that Americans are so untrusting of Mormonism since it's such a quintessentially new world religion. You'd think that a ruthlessly capitalist and hierarchical brand of Christianity where the center of God's kingdom isn't someplace in the Middle East but the United States would appeal to American exceptionalism. They even banned black people and called them cursed by God until the 1970s. Mormonism embodies the American zeitgeist more fully than any old-world religion, it's a wonder it isn't more popular.

Maybe it's the magic underwear that creeps people out?