Technically, SRB2 is a 2.5D game, unless you're using models.
A 2.5D game is actually just a 2D game where you occassionally move to the background or foreground without ever leaving the 2D plane. Most games that are like this usually have 3D assets for the background while characters are 2D sprites - much like the original Klonoa on the PS1, or Tombi.
SRB2 is a 3D game with 2D sprites that are drawn from various angles. As for this Mario, while yes it could be very neat to see this Mario from multiple angles, I can understand not doing it and just trying to give it a cool gameplay gimmick.
I do have to mention that there used to be an NES-Mario.wad back in final demo that did have rotations. Granted, it was pretty choppy since it is pretty old now, but I would suggest digging around to find that and at least use it as a base for multiple angles, if you're inclined to use them.