"The Mormon faith began because every Christian faith was corrupt and not true according to Smith."
Actually, the fact that Smith became disillusioned with Christianity at the time, is not when it began.
It began, when he decided to write the book of Mormon.
It went further than that. Joseph Smith's first version included the "revelation" of the restoration...which was based upon the spiritual entity that appeared to Joseph telling thim that every creed of Christendom was an "abomination." (The "corrupt" reference was to the "professors" of Christ...especially indicting all Christian leaders).
So, an LDS person who holds to all aspects of the original vision for the LDS church (does this include Romney?...don't know it would be good for him to answer this) would hold that all of Christendom's creeds are an abomination before God and that its leadership is corrupt.