They've still got a while before the game's release, I doubt they'll be just working on the making new characters & items 'till then, I'm sure there will be slightly graphical enhancements. Besides, the SSB games are always pretty zoomed out so you can see the whole stage usually, I don't think people are going to notice a tree that looks pixellated close up if the screen is far away. It's the same as on those games where when you look really close to a wall or floor, you can see the pixels 'cos they didn't program the textures for close up or something. Hard to explain.
The character models look very nice, considering the game won't be released for 3/4 of a year, I don't think the graphics are going to get worse