Modern mainstream religions could be considered "cults", and every form of Atheism could as well. But why should it matter if that particular term can be applied?
Why would we want to call every modern mainstream religion a "cult" apart from a desire to provoke some type of defensive emotional reaction from religious persons? And why would anyone be interested in manipulating such a reaction?
If a perspective is true, if it genuinely and accurately reflects reality, why would anyone want to provoke a defensive emotional reaction from someone who holds to it?
Encouraging the use of a word like "cults" (with its negative connotations) to describe all religions seems to me to be nothing more than what's known as "spin": give a perspective a label with negative connotations and maybe people will reject it, regardless of whether or not the perspective is true.
It's very easy to "spin" things. Lawyers and politicians do it as a matter of course. What one calls courageous another calls foolish; what one calls disciplinary another calls abusive; what one calls devoted another calls narrow-minded. Manipulating people psychologically or emotionally is often an effective way to control and influence them, but it doesn't serve to reveal truth.
To more directly answer the question, though, the word "cult" has a number of definitions, and some are vague enough that they could indeed describe any worldview whatsoever, including every brand of Atheism.
1. CULT - intense interest in and devotion to a person, idea, or activity.
Certainly Atheism is devoted to an idea: the idea that there is no God. Therefore, Atheism is also a "cult".
2. CULT - a quasi-religious organization using devious psychological techniques to gain and control adherents.
Keep in mind that a "religion" can be defined, according to dictionary.com, as "a specific fundamental set of beliefs and practices generally agreed upon by a number of persons". Under this definition, any brand of Atheism is "quasi-religious". Since Atheism is necessarily false, a common way for it to gain adherents is to practice "devious psychological techniques", such as using a loaded word (like, oh... "cult", for example) to describe theism. So certainly, by this definition, Atheism is clearly a cult.
3. CULT - an instance of holding in great respect a person, ideal, or thing, especially as manifested by a body of admirers.
Again, Atheism clearly fits in with this definition too.
Atheism is not the only view that could properly be called a "cult", though. As I said initially, the word "cult" has a number of definitions, and some, like the first and third ones listed above, are vague enough that they could describe any worldview at all.
However, when I personally hear the word "cult", what I automatically associate it with is the following definition...
4. CULT - a religion or sect contrary to Christianity, with members living cut off from the rest of society under the direction of a charismatic leader.
With that definition, only certain small groups would properly be considered cults. Christianity would obviously not be among them, and in general, Islam, Hinduism, and Atheism would also not be among them.
In sum, while the word "cult" has a variety of meanings and could theoretically be accurately applied to any worldview at all, trying to manipulate people into rejecting a worldview by giving it a label with negative connotations is nothing more than a devious psychological "spin" technique.
Whether or not we label Christianity a "cult" won't change the fact that Christianity is necessarily true.