In Brawl's time, Paper Mario had two games, one of which was pretty new and was considered a masterpiece and a third game coming up that was released during Brawl's development and got a bunch of Trophies.
And yet Dr. Mario was planned for return while Paper Mario was skipped over.
Only thing since then was a mediocre fourth game. One that manages to provide an excuse to make Paper Mario a clone.
Doesn't edge out over the Smash veteran that was planned to return.
He is a spin-off character.
So is every ****ing Mario character that isn't Mario himself and possibly Luigi depending if you count Donkey Kong as the start of the Mario series or Mario Bros.
Try again.
EDIT:
Also, Dr. Mario.
And the often discussed Paper Mario.
By the logic of "non-platformer characters", they're spin-off characters.