Octagon Girls aren't an innovation of the Ultimate Fighting Championship; models have long been used to announce the rounds in boxing and wrestling events. However, the UFC takes those sports to the extreme. A 2013 article in Psychology Today pointed out that it's especially problematic to have women on display for sex appeal in the center of a ring that exists purely for aggression. In that sense, UFC Octagon Girls might be some of the most egregious examples of sexism in professional sports.
Sexism in the UFC isn't limited to the presence of ring girls, however. The organization does have female fighters, although they, too, allege inequality. In 2015, UFC star Ronda Rousey actually complained that the ring girls were overpaid, in terms of how much they earned compared to female fighters. Ring girls are paid about $20,000, and Rousey alleged some fighters were paid less than that (though she herself has earned millions for a single fight). Rousey did suggest that the real problem might be the pay of female fighters as compared to male fighters, but her decision to call out the Octagon Girls, specifically, could be seen as odd.