Eh, I don't think it would be breaking any rule. I've seen new players on other boards do it before (if I'm wrong, someone let me know).
Also, I've never been to an official tourney before (busy as all get out usually) but that sounds like its bound to happen from time to time. Don't let it get you down! Some people say all sorts of things when they win like that. Ness has been kinda shafted in pretty much every single Smash title so far. In this one, he's getting his time to shine; many people say he's top 5 material. But regardless of how good he is or isn't, it doesn't really matter. Take Shaky in Brawl. He mained Ness, which was pretty much considered suicide in tournaments. But he did well and was well respected for it (some people said he was the best Ness). There's something to be said for sticking to your main regardless of how good people say they are.
(Plus, Sheik. Any character is bound to look 'not that much good' to a Sheik. Bleh.)