After having played over 500 battles online consisting of both For Fun and For Glory matches, and even watching others battle via the Spectate mode and Replay Channel, I feel it's safe to say that little Mac is the single most played character in the game! Now overall there's a pretty good amount of character variety online with no one character being used just as much or more than every other combined, but naturally there are some characters used more or less than others, and Little Mac is used significantly more than the next most used characters. As for everyone else, I put together a list showing how frequently they've popped up. Of course all this is just from my experience with the game, so if you were to make your own usage list it'd probably be a bit different, but I think we can all agree that Little Mac is used the most! Also the list is purely about usage, not how good they are, afterall Little Mac is at the top! 
OVER-USED
Little Mac
OFTEN-USED
Mario
Kirby
Robin
Ike
Zero Suit Samus
Shulk
Mega Man
REGULARLY-USED
Luigi
Peach
Yoshi
Donkey Kong
Link
Zelda
Ganondorf
Samus
Pikachu
Charizard
Jigglypuff
Greninja
Ness
Captain Falcon
Villager
Marth
Lucina
Duck Hunt
Sonic
UNDER-USED
Dr. Mario
Bowser
Bowser Jr.
Diddy Kong
Sheik
Olimar
Pit
Dark Pit
Paluteana
King Dedede
Meta Knight
Lucario
Fox
RARELY-USED
Rosalina & Luma
Wario
Toon Link
Falco
Wii Fit Trainer
R.O.B.
Mr. Game & Watch
Pac-Man
NEVER-USED
Ice Climbers
