Honestly, I don't think there's a single reason to act cocky online at all. If we talk about FG in particular, the environment is actually kinda stale since there's basically no stage diversity and the fact that there's no real matchmaking as far as I know only serves to push that point further.
People are so quick to defend their ego in FG when FG barely means a damn thing in the bigger "How good are you at Smash?" picture. Not only that, but cocky players have made a habit of tarnishing certain characters and actions by turning them into jokes that only players trying to 'disrespect' play or do. You just end up with a scene where people feel justified to act like asses because "You play ___." or "You went for ___ too many times." or "You taunted once." and judging people on a non-competitive level for how they play a game (Good or bad.) is frankly just stupid.
You can be proud of yourself by all means, but there's a fine line between acting like a cocky degenerate and actually evaluating when something was good or not that a lot of people just can't see, and the second you inflate your ego is the same moment you're taking the game at what is effectively a non-competitive level too seriously.
(Oh, and for the record, if you are proud of something, one taunt will do, thanks... Kirby's on the spot walkdance is also acceptable because it's frankly adorable and you might not want to waste your copied ability. :V)
(Oh, and swapping between G&W's Up and Side Taunts is acceptable too, but shhh, don't tell people there are exceptions. Protect the sacred Bell Taunt combo.)