It's not just voice acting that's important. It's voice directing that's equally important, if not, more so.
And I say that's its even worse in this game than it was in Brawl and it really screws over the VAs, especially the American VA's. And it shows with all the characters(fighters AND assists) here.
The Mario cast sounds less enthusiastic and lively. They sound muffled and soulless, especially the Mario Bros. If you like how they sounded in their own series and spinoffs(I LOVE the taunts in Mario Golf: Toadstool Tour), you're not going to like them here. And I don't care what Ryu Myuutsu says, I HATE the roars for Bowser, DK, and Diddy!
The Zelda cast sounds weak when compared to their source games and even Hyrule Warriors.
Samus Aran's dialogue is generic and short, but you wouldn't know since you can barely hear her. Samus need a good American voice direction, so she, I would think, can sound as good as
this
Kirby sounds almost exactly like he does in Triple Deluxe. The problem here is that it doesn't really work in Smash. He should be a bit more vocal . Another problem is that he really sounds incomprehensible. He was easy to understand in the original Smash and even Kirby: Right Back At Ya, but it seems like they're trying to make him sound more babyish and unpronounceable.
And Speaking of Kirby: Right Back At Ya, I would love for King Dedede and Meta Knight to have their roles reprised by Ted Lewis and Eric Stewart(or at least, a good expy) respectively. I was never a fan of King Dedede's... uh, "quacks?" Also, its really unfair that DDD doesn't get to speak properly, yet Meta Knight(whose voice I find generic. I rather he sounds like Antonio Banderas) does? Oh course, he's Sakurai's favorite(well,he was).