145 is the heaviest weight-class in WMMA, not 135, which is what Ronda competes at in the UFC.
Part of Conor's appeal is his fighting style, but the most important aspect of his appeal though, is that he's the most interesting, controversial, and entertaining personality in combat sports right now alongside Rousey, and Mayweather. If Conor Mcgregor had Cain Velasquez's personality, he would not have been setting records for viewership ratings, ppvs, live gate, nor would he be getting all this coverage.
There are a lot of active fighters at 170lbs and above that have entertaining fighting styles, like Chris Weidman, Cain Velasquez, Junior Dos Santos, Anthony Johnson, Robbie Lawler, Matt Brown, Carlos Condit, Yoel Romero, etc... the list goes on and on, but none of those guys are even half as popular as Ronda/Conor because they all lack the rare combo of charisma, and attitude that is most important to attract the attention of the masses. This isn't the 20's-90's anymore, mainstream audiences are attracted to fighters who have personalities that are larger than life, as much, if not more their fighting style.