Answer:
No. What they preach is truth according to their own doctrines.
However, the vast majority of the Christian Church regards some of Mormon doctrines as incorrect, and even heretical. This is in part due to doctrines to do with salvation, and the acceptance of the Book of Mormon as scriptural.
The Christian Church worldwide regards Mormonism at best a fringe church within the Christian umbrella, but certainly not a mainstream Christian denomination. At worst, some even regard it as a sect or even a cult, although this, I believe, is a little unfair.
So, when Mormons preach their doctrines door-to-door, as far as they are concerned they are 'truth' but the vast majority of Churches across the world think otherwise, and not without reason, if the history of the Church, theology, Biblical textual criticism, Biblical archaeology and so on are taken into account.