Technically, neither. Monotheists believe there is only one god
and worship only that god. Polytheists believe there are multiple
gods and worship multiple gods. The Church of Jesus Christ of
Latter-day Saints (the "Mormon" church) doesn't exactly fit either
of these descriptions.
Technically, Mormonism is Monolatrism. Monolatrists recognize
the existence of many gods, but worship only one. Mormons believe
that God the Father, Jesus Christ, and the Holy Spirit are three
seperate beings. They also believe that it is possible for humans
to eventually become gods through eternal progression. However,
while they recognize the existence of multiple gods, they worship
only one - God the Father. Worship of or prayer to any being other
than God the Father is considered blasphemy.