I understand Mormonism is polytheistic because it teaches that the Father, Son, and Holy Spirit are three separate gods. When I recently told a pair of Mormon missionaries they were polytheists, they heatedly denied it, claiming their church teaches the doctrine of the "Godhead" of the Trinity and worships only one God. What's up?
The answer to your question is: Yes, Mormonism is essentially polytheistic. But let's make sure we're straight on our terms. In Catholic lingo, "Godhead" is another way of saying Trinity in that we understand the Godhead as one God comprised of three divine Persons--not three distinct gods. Polytheism means the worship of a plurality of gods.
Some Mormons, recognizing their theology is polytheistic, prefer to soften it by referring to their religion as "henotheistic," which means the belief in many gods but the worship of one chief god.
For Mormons, "Godhead" means a spiritual partnership that exists among the three "gods" of this planet: God the Father, Jesus, and the Holy Ghost. (Mormons usually don't say "Holy Spirit.")
Joseph Smith, the founder of the Mormon Church, declared: "I have always declared God to be a distinct personage, Jesus Christ a separate and distinct personage from God the Father, and that the Holy Ghost was a distinct personage and a spirit; and these three constitute three distinct personages and three distinct gods" (Teachings of the Prophet Joseph Smith, 370).
In fact, Mormonism teaches that God the Father is essentially a "perfected man" and is limited by a body of flesh and bone and thus is limited to time and space. He happens to reside on a planet near an uncharted star called "Kolob" (Abraham 3:3-9).
Since Mormons worship both God the Father (Doctrine and Covenants 18:40) and Jesus Christ (3 Nephi 11:17; 2 Nephi 25:29), and since they believe the Father and Jesus are two separate gods, they truly are polytheists.
For an added, bizarre twist, turn to the Pearl of Great Price to see this doctrine contradicted: Moses reportedly says, "For God [the Father] said unto me: 'Worship God [the Father] for him only shalt thou serve'... Call upon God [the Father] in the name of mine only begotten [Jesus] and worship me'... Depart from me, Satan, for this one God only [the Father] will I worship, which is the God of Glory" (Moses 1:15-20).