I haven't seen the movie, but I wouldn't be surprised if it made it seem like his actions were more significant than they were. Flynt and Hustler didn't come along until the sexual revolution was well underway and rode on the coattails of and tried to be more explicit and controversial than Hefner and Playboy, who had been around for over 20 years. Flynt basically ended up being significant only because he allowed an offensive ad to be published, was sued for defamation and won the case, setting a precedent for free speech cases to come. It doesn't seem to me like he deserves a whole lot of credit.
Hefner was much more prominent in culture because he started Playboy before the sexual revolution and helped spark it and because he and it were more acceptable to the public. The magazine had pictures that were relatively modest and high quality articles and interviews, not to mention short stories by well-known authors. It was a joke that one read Playboy for the articles, but there was a little truth in it because the magazine's written content and popularity did help legitimize it in culture. Also, the lifestyle that Hefner represented was attractive to men who similarly wanted to be rich and surrounded by women. Playboy was sort of a lifestyle magazine for men and there was less shame in reading it. Hustler, on the other hand, was pornography and deliberately controversial. Flynt also wasn't attractive and was confined to a wheelchair. He and his magazine came across as dirty and sleazy. In fact, they may've even helped Hefner's and Playboy's reputations and prominence in culture, because the latter ended up looking even classier and more glamorous in comparison.