Since Mario Kart 64 stuck with the Pipe Frame kart for everyone, I imagine that it would indeed be more polygonal since its 3D design commands more surfaces, compared to the simpler car, plane or boat designs in Diddy Kong Racing. You have to remember that the other Mario games for the N64 only have the character themselves, and if they have items, they have less polygons overall (example: tennis racket, golf club). Another thing to note is that Mario Kart DS doesn't have a Pipe Frame kart. I think it's one of the factors that prevent it from being full 3D for the time being.
Thanks for reading.
Thanks for reading.