Every time I'm beaten by a legitimately good play, I always send a "gg" their way. I feel like while there are definitely some cheaper attacks/characters, in general when you lose, you lose because you were out-played, not because your opponent had an unfair advantage over you.+ 1. When I find one of these: Dashboard. Avoid said gamertag. I might reduce my chances of finding a match by 50% but I'm not fighting those button-mashing lag-abusing post-round-taunting scum twice.
On the bright side: I fought Master Ari on ranked. He won 3-1. I sent him "GG" and he answered "GG very nice". Why can't everybody be this civil? Being beaten like this is a pleasure.
That said, I have issues with taunters and particular spammers, though I never harass them. I simply grumble to myself and hope I don't have the misfortune of fighting those guys again.