Auroville is the product of a fringe spiritual movement, but not an outright cult. Back in the 1960s, Auroville was able to attract so much approval from representatives of various governments and the NGO world because the Mother and Sri Aurobindo were seen as relatively reasonable people, in spite of their utopian visions. As far as fringe spiritual movements go, Auroville is comparable to Rudolf Steiner, Gurdjieff, the spiritual side of the Esperanto movement, etc. which definitely have their share of critics but are large, institutionalized phenomena where people can enter or leave the movement as they please, and lives aren’t completely controlled and subjected to the whims of a single living leader or a committee thereof.
Also, when you actually visit Auroville, you find that it has splintered into disparate communities that sometimes outright dislike one another. Again, this seems very different from cults, which tend to insist on forced unity and orthodoxy among its membership.