Latency on wireless controllers isn't a big deal (and a lot of Smash players are using wired Gamecube controllers anyway), but it's not a big deal on wi-fi either. The problem with wi-fi is packet loss and not being able to send and receive at the same time, which feels like latency in fits and starts, because it has to wait until the packet sends successfully. Ethernet helps with Smash, but it still sorely needs rollback netcode regardless. Even on a wire, you're still on delay-based netcode.