I'd say a lot of the backers are bronies or former bronies. That's just how it goes considering the TFH's precursor. A fair few migrated over, and they're generally the ones that spread the word about the game and would naturally be among the first to buy/back it. To most everyone else it's some random fighting game that has cute quadrupeds instead of musclebound guys or skimpily dressed women, or what have you. It's an unknown property and a lot of people would just overlook it either because the aesthetic doesn't appeal to them, or they'd rather not take the risk of spending money before the game comes out and there are reviews. As the game gets closer to release, and is more polished and presuming it gets advertised enough I would imagine there will be more and more people who have nothing to do with the pony fandom getting interested.
Once it's out and (presuming) there are good reviews of it, there will be a large influx of people buying. I wouldn't be surprised if in the future, especially if the game ends up becoming a series, the non-pony fans (former or current) will end up outnumbered.