Awesomely stupid indeed; the dude can store projectiles in his pocket!
Regarding a possible subspace mode.. I was looking at the trailer again and while I believe the Wii U footage is just custom scenes for the trailer, like Brawl, I think there may be an adventure mode in the DS version.
I can't link the video now (lazy/mobile) but there's a clip of Mario jumping by himself in what I originally thought was a dkcr inspired level, but the design felt off and didn't match anything from other series known to be represented in the game. What really stood out as odd was simply the fact that the scene only shows Mario and no other character.
I'll link the clip when I'm home..