It can be suggested that its popularity helped spread the culture, but it did not create it. New York City had an underground Disco culture before this movie, but it wasn't necessarily like the movie depicted it. The movie was based on "The Tribal Rights of the New Saturday Evening", written by Nik Cohn and published in New York magazine. Here's where it gets a little complicated. In his attempts to research this underground movement for his article, Cohn went to a club in Brooklyn when upon stepping out of his cab there was a fist-fight happening in front of him. Basically, his attempts (he tried more than my described attempt) to penetrate this scene seemed too dangerous for his liking so much of his article was fabricated, at the best basing the attitudes and dress from what he witnessed in parking lots or heard third hand. So there was a definitely a culture and interest in it, but there is a possibility that the culture reflected in the movie was inaccurate before it went mainstream. My source for this is "Stayin'Alive: The 1970's and the Last Days of the Working Class." Chapter 7 The Important Sound of Things Falling Apart. By Jefferson Cowie. I apologize for the brief explanation and crappy citing format, but I am at work and on mobile.

The first disco in the US opened in 1960, basically a direct copy of the French discotheque dance clubs, and they were relatively elite hangouts until the late 1960s. The "disco culture" that was belatedly popularized by the 1977 film was itself a watered-down version of what by the early 1970s had become an important part of the post-Stonewall gay culture in New York: hedonistic dance clubs that attracted a gay male clientele in particular.

Peter Braunstein, writing in American Heritage (Nov 1999 v50 i7 p43), claimed that

"For the first half of the 1970s, disco was an extended conversation between black musicians and gay dancers. These were halcyon days for those in on the disco craze while it was still a glorious secret from journalists and most of the white, heterosexual world, before it became, as Andrew Holleran put it, "another possession of the middle class." As the Hustle craze of 1975 elevated disco to a national phenomenon and enticed straight white people to boogie, a clash of cultures occurred. A mere half-decade after Stonewall, heterosexuals were being forced to navigate in an inverted cultural landscape whose terms were set by gay men. Dancing Madness, published in 1976 to capitalize on the disco craze, set down some guidelines: "The first test for the hetero male who wishes to be in tune with the basics of bisexual chic is to not feel threatened when addressed as baby' rather than sir.'" A reporter from Harper's, dispatched to the disco front in 1977, noted the nuanced stratagems necessary for straight entry into the world: "Now middle-class young men cruise the banquettes each weekend, but the tone of the place is sufficiently gay that a woman can protect herself by adopting a fierce gaze to indicate dykishness, or by staring fixedly at herself in a mirror, for self-absorption is respected here.""

The coverage in the mainstream media of the late 1970s misses most of that history as it only picked up on disco once it became commercial and mainstream-- about the same time the movie was produced, in the wake of the cultural phenomenon that was The Hustle. Time, Newsweek, US News, and similar outlets wrote about disco c. 1977 as if it were new, and completely overlooked the link to the gay male subculture while emphasizing hedonism, drugs, and fashion among heterosexual patrons of clubs the gay patrons had already moved on from.

Generally speaking, I've always felt that once a major studio makes a film about a cultural phenomenon it's long past its peak importance. Saturday Night Fever is certainly a case in point, as while there was still a lot of money to be made in disco in 1978-81 (Studio 54 was sold in '81) mainstream disco was a pale shadow of what came before in many ways, largely corporate, largely hetero, and sold to teens in the midwest as a form of mild rebellion.