I watched the direct again, and you can see these mirrored stances there as well, but as others have said, it's definitely not for everyone.
So far, from what' I've seen:
Mirrored:
Bowser
Charizard
Greninja
Kirby
Mario
Mega Man
Not Mirrored:
Diddy Kong
Donkey Kong
Fox
King Dedede
Link
Little Mac
Lucario
Marth
Olimar
Pikachu
Pit
Rosalina
Samus
Sheik
Toon Link
Villager
Wii Fit Trainer
Yoshi
Zelda
Zero Suit Samus
Unknown:
Sonic
Peach
Luigi
(a funny one to note is Rosalina. In all the promo pictures for her, like on the Smash Bros site, she's right handed, but in the game, she's left handed)
Of course, some of these could still change, but I believe that the mirrored stances only pertain certain characters where their actions aren't really affected by which hand they're using. Link and Samus, as an example, are historically left and right handed, respectively, so they aren't changing these characters' stances. Plus, most of the characters who are not mirrored face directly to the side anyway (with a few exceptions like Marth), so it's doesn't really affect them. But other characters, like Mario and Bowser, who primarily face the camera, but can hit with either hand, are mirrored so that you're not seeing their back half the time. This makes sense and is actually a really cool feature, and it's also nice to see Sakurai going the extra length to make sure characters who it would affect negatively are instead staying true to their character.