Online is horrible.
And the best characters on wifi are Wolf (his fsmash practically breaks the game of wifi mixed with lasers/bair, has a fast roll and little lag on smashes = spam), Snake (his explosions cause lag, his roll is also fast, and spamming tilts work).
Pretty much every character without a projectile gets screwed over. People who play wifi and wifi only just camp with projectiles. Chars like MK and Marth just can't get past them effectively. You can't perfect shield, you can't punish spot dodges/rolls, and you can't space well either. The best thing MK has on wifi is his tornado, and his tornado only.