I have played this game at the GDC.
The normal view is fine, and it looks like any other Mario Tennis game but with 3D effects because it's on the 3DS.
However, when the 3DS is raised up, the view switches to a behind the shoulder view, and the 3D effect disappears. Very odd. I suppose it's still early and maybe they could add the special effects in for that view in the final version.