Are you just using textures or are you using anything else that may affect the game (import stages, Brawl+, PSAs, etc.) The textures shouldn't kick you off Wi Fi (whoever you're playing won't see them mind you) but the other stuff will kick you off unless your opponent has the exact same thing.
I don't play on imported stages I only play on stages with just slight changes and I also use character Textures. I can understand that if I use something that can take advantage over Wi Fi, I can get booted or possibly banned if I were to do something really stupid. But I just don't understand why it disconnects me its most likely happens when i smash the Smash ball