Justin Wong dominated in a game that had little international competition, a small community compared to games of today, and before the information age really took off.
His domination isn't as impressive as Zeros to me, especially in a game like Smash 4 that's so bad. Hard to be consistent in a game like that.
Has Justin ever won a MvC2 tournament with almost 2000 entrants?
But he dominated when the game was already at it's peak in terms of skill and knowledge of the game. Justin can use low tier characters and still do more than most people. Only way to even compare Zero's dominance with Justin's is if he kept it up for a few more years. You are going to crown the guy best ever in anything when the game is barely over a year and more than half of the top players from previous games don't even give a crap about.