There's probably some level where bad aesthetics will turn people off from playing a fighting game, but it's pretty much an ancillary feature and I'm not aware of any game on the market that has actually crossed that threshold. If the core gameplay isn't there then the game will die. If the game is losing momentum it's always down to the gameplay and no injection of good visuals will do anything to move the needle. Just look at DNF Duel. That game had gorgeous visuals and some of the best rollback netcode known to man, but the gameplay just wasn't strong enough and it died a pretty meek death.