They do not skew the mainstream teaching of Christianity, they wrote the tenets basically. So you don't have a gotcha. I bet your fundamentalist sect considered the LDS a cult.
And I bet the Jews in Israel thought that Christians were a cult, what is your point? At one point they would have been a "cult" if you use the "skew" portion (it isn't there in the dictionary, but heck we'll have fun with it anyway). But not once they have enough followers to be considered mainstream.
The reality is they worship the same god as the Jews, Christians, and Muslims. While they aren't Christian and their religion is different from Christianity, so it is with Muslims, and Jews. Having some different precepts isn't what makes a cult, it is belief in a person or thing rather than a god (first definition), or the number of followers (definition 2) as well as others thinking they are "sinister".
The number of followers for the Mormons simply overcomes the "relatively small number" property of a cult.