Well, the Mormon church wouldn't even be in existence without Christianity -- it's an offshoot. And what is your source for your first sentence?
And Joseph Smith, founder of the offshoot religion didn't claim he was going to be raised from the dead, whereas Christ did. The resurrection of Christ is the whole essence of Christianity and it doesn't make sense for it to grow so quickly if it didn't happen -- especially when Christ's followers exhibited cowardice and uncertainty before his death. On the other hand the principles of Mormonism could go on regardless of whether Joseph Smith existed or not. His existence was not central to the religion's principles as is Christ's resurrection central to Christianity.
And the Mormons had to flee to the middle of nowhere (Utah) to grow substantially (and also grew from within because of polygamy) whereas Christianity grew within the dangerous Roman empire its first 300 years.
But the fact Mormonism did grow can be considered "some" evidence or indication of its possible truth. Of course this fact alone wouldn't be enough to convince me, but it is more of a positive for the truth of Mormonism than if the church declined in numbers. There are many other factors that are needed to add weight to the totality of evidence but the growth of a religion can be considered some weight. The importance of that weight is subjective.
The rapid growth of Christianity in the dangerous Roman Empire in itself would not be enough to convince me of its truth, but the existence of that fact does support my belief more so than if Christianity didn't grow much. Therefore it meets the definition of evidence (see post 13) because it is a thing that is helpful in my forming a judgment.