"Cult", in our society, is a pejorative term, bringing to mind images of drinking the Kool-Aid and Charles Manson. However, it is a perfectly fine academic word, and is used to describe religious organizations of almost any ilk.
Here is Merriam-Webster on the topic:
: formal religious veneration : worship
: a system of religious beliefs and ritual; also : its body of adherents
: a religion regarded as unorthodox or spurious; also : its body of adherents
: a system for the cure of disease based on dogma set forth by its promulgator
a : great devotion to a person, idea, object, movement, or work (as a film or book); especially : such devotion regarded as a literary or intellectual fad b : the object of such devotion c : a usually small group of people characterized by such devotion
See: a perfectly good word, used by religion scholars everywhere. And, as one can see, it applies to nearly everyone, from us Catholics to voodoo adherents to Moonies to Muslims.
The questions seems to be whether or not Romney is a part of some sort of dastardly organization that does harm to its members and society in general. While I believe much of the Mormon religion is wrong both in content and practice, I don't believe it's evil or is plotting to harm the United States by setting up one of its own as leader of the free world.
Let's just all take a breath, shall we, and think about the words we use, and how we actually mean them. If you want to judge Mitt Romney on his faith, go ahead, but don't be hateful about it, even in a back-handed way.