As for the status of Joseph Smith, founder of the Mormon Faith, he is not considered by Bahá’ís to be a prophet, minor or otherwise. But of course he was a religious teacher sensitive to the spiritual currents flowing in the early 19th century directly from the appearance of the Báb and Bahá’u’lláh and the Revelation of Their Messages of hope and Divine Guidance. In this respect you might find chapter ten in the late Hand of the Cause George Townshend’s book, ‘Christ and Bahá’u’lláh,’ interesting.
(Universal House of Justice, Lights of Guidance, p. 511)
Joseph Smith we do not consider a Prophet, minor or otherwise. Certainly no references he made would have foretold the coming of the Revelation in his capacity as a Prophet.
(Shoghi Effendi, Lights of Guidance, p. 484)
Regarding your question concerning Joseph Smith and the ‘Book of Mormon’; as the Bahá’í Teachings quite clearly outline the succession of Prophets from the days of Christ as being Muhammad, the Báb, and finally Bahá’u’lláh, it is obvious that Joseph Smith is not a Manifestation of God.
(Shoghi Effendi, Lights of Guidance, p. 510)
The Guardian would advise you to teach the Mormons like everyone else, the Faith, when you find them receptive. They have many good principles, and their teachings regarding chastity, not drinking or smoking, etc., are quite similar to ours, and should form a point of common interest."
(From a letter written on behalf of the Guardian to an individual believer, August 18, 1942: Bahá’í News, No. 416, November 1965)
The Mormons are a people with high principles and ideals, and the step spiritually into the Cause is not as difficult for them as for many others not possessing their faith and devotion. However, the very zeal with which they serve their own Faith makes it difficult for them to grasp the greater vision of our Holy Cause.
(Shoghi Effendi, Lights of Guidance, p. 510-511)