Masahiro Sakurai, the director of the Smash Bros. series, has told IGN that there’s “no plans” for DLC for Smash Bros. on the 3DS or Wii U at the moment. Instead, he’s focusing on making the games “the best and the fullest experience possible”.
Even so, DLC is “the type of thing [they] could take into consideration” once the games are finished. It’s simply not being thought about yet. I’m sure we’ll actually see quite a bit of DLC once the game’s released, but I do believe Sakurai when he says he’s completely focused on the core game for the time being.