Every previous Smash had completely different art styles, while Smash 4 is basically just a brighter Brawl, to make it line up with Nintendo's other games.
First, I think Nintendo's decision to go for closer art styles in most of their first party games was stupid in the first place. Mario Galaxy was clearly a Mario game in spite of the different lightning and background details from other titles. They didn't need to make them look even closer.
Second, Smash's adaptation to that artstyle was lazy like previously mentioned. Even Melee and Brawl, in spite of being in very close hardware, looked vastly different, so there's no reason they needed to stick with Brawl's art style, but more colorful. Unless this brings some advantage in the end (like a much higher number of characters since they didn't need to be completely redesigned), it's a fairly disappointing look.
Now, obviously, the 3d models were improved and such, but looking at them up close still shows a bunch of imperfections and visible polygons, and, from far away, the difference won't be big anyway. It just seems like a bad choice.
