I wouldn't call Super Mario Galaxy 2 a real 3D Mario game since it follows a linear level design. I say the last sandbox open world 3D Mario game Sunshine.
Anyway I agree. 3D world looked lame and played lame It was a sequel to a not very good 3D Mario game which hurt it even more. I felt the game was lacking any sort of passion that the the Galaxy games had. Odyssey is the complete opposite. Not only does it have some real passion behind it but it also is a sandbox Mario game which is something that needed to return.