Is Mormonism a Cult?

In a certain sense, no. That is, if by "cult," you mean the sort of mesmerized, secretive organization which demands unquestioned allegiance to the leader and which leads to the deaths of many of its adherents, a la Jonestown, David Koresh, etc. then Mormonism isn't a cult in that sense. At least, not today, though even cursory reading about early Mormon history certainly leads one to conclude that Joseph Smith and Brigham Young were more akin to Koresh than to Paul or Jesus.

But moving over to the theological realm, the answer is certainly an emphatic "YES." Consider the following:

1. Mormonism was founded by Joseph Smith, who claimed that he was told via direct revelation that he needed to found a new church of "Latter Day Saints' specifically because all other churches and Christian denominations were false and corrupt. Thus, their differences with historic Christian orthodoxy are not incidental, but central to Mormons' self-identity and reason for existence.
2. Mormonism rejects the unique authority of the Scriptures, and considers them their inerrant nor complete, adding to them not only the Book of Mormon, but also Doctrine and Covenants and The Pearl of Great Price.
3. Mormonism emphasizes the continuing nature of revelation through official prophets. Through the Mormon hierarchy of President, First Presidency, Twelve Apostles, First Quorum of the Seventy, and Second Quorum of the Seventy, Mormons can receive authoritative interpretations of both the Scriptures, the Mormon additions, and entirely new authoritative revelations. It is uncharitable to point out that some of these "new revelations" have come about because of the changing of social mores or desire for social acceptability in the wider culture, but with issues such as polygamy and the admission of blacks to the Mormon priesthood, such certainly seems to be the case.
4. Mormonism affirms a primordial spiritual existence before birth as God the Father's spirit sons and daughters, who receive bodies when humans procreate here on earth. How the first humans got their bodies I do not know, since there seems to be a need for a first set of bodies for the Father's spirit children to inhabit, but whatever.
5. Mormons are non-Trinitarian. They affirm the Father, Son, and Spirit as unity in purpose and mind, but not in essence, and such unity as there is not eternal. Moreover, Mormonism is explicitly polytheistic, with Brigham Young teaching, "How many Gods there are, I do not know. But there never as a time when there were not Gods and worlds."
6. To Mormons, Jesus is Redeemer, but his deity is derivative and lesser than that of God the Father.
7. For the Mormon, humans are not inherently sinful. They do not possess an innate sinful nature, but are basically good.
8. Mormonism teaches that eternal reward can come to Mormons by their own efforts. Salvation is thus essentially not by grace, but by works.
9. Mormon salvation means that good Mormons ascend to the highest level of reward (the Celestial Kingdom), where they and their spouses (to whom they are still married for eternity!) continue to procreate as Gods, whose spirit children will one day inhabit other worlds. Less good people, who aren't quite righteous, go to the Terrestial Kingdom, where they don't suffer, but also aren't ruling as gods. The Telestial Kingdom is for the wicked and includes suffering. And finally, the Devil and fallen angels are confined to the Lake of Fire.

That is not even an exhaustive treatment of Mormon theology and its departures from historic Christian orthodoxy. But it is quite enough to say that while Mormonism is something, it is not "Christian" in any recognized theological sense of the term. At best, it is a religious movement which incorporates some Christian terminology and uses the Christians Scriptures. But it is not inaccurate in the least to label it a "cult."