The rendering method for the game is still 2D, not 2.5D.Why does the 3D suck ass in this game?
3D models are used for the characters but everything each is pre-rendered graphics, 2D bitmaps.
They use these very effectively actually, for coin animations or anything that rotates or scales, these graphics always look smooth and detailed.
Back to the point, the fact the game is more than likely using the same 2D engine from NSMB Wii is what limits the possibility for a strong 3D effect, instead they just made a blurring effect for depth.
Other 3DS software also adjusts the image rather than the depth as you use the 3D slider, 3D Classics Kirby's Adventure for example does not just depth, but darkens the background.