It doesn't matter if you beat them 10-0 online. Despite the fact that netplay can often be lopsided, where one connection is better than the other (one player has a lot of delay where the other player doesn't), there's a lot of variables that come into play when playing online. Also, I've had some hellish lag spikes in SF4, where my opponent showed me the replay he recorded and he had no lag at all.
I suck online and I can't adapt to delay/lag like other people can. Some players regarded as the best in American in UMK3 would beat me convincingly online. And when I played them in tournament, I beat them 2-1, 2-0, etc. One of them even said to me that he now sees how much delay affected me and that I was a lot better than he thought.
The only way to prove you're better than another player is to play them offline. I didn't mean you can't prove you're good online, that was my mistake. You CAN tell that someone knows their shit if you play them online. But you can't say "I beat you online so I'm better than you", that is something you can only prove offline. That's why Curbo and the other online players that now do well in tournaments no longer get flack. They've proven themselves at tournaments.