The Paper Mario stage is named "Paper Mario", because the stage switches between several locations from the Paper Mario series. It's not one specific location. So far we've seen the grassy area from Sticker Star, SS Flavion from Thousand Year Door and now we know that Bowser's Castle is one of the locations too. Same goes for 3D Land. Those stages are named after the games because they're based on the games themselves instead of being based on a specific location.
It's not like Gerudo Valley, Arena Ferox, Rainbow Road or Spirit Train that are directly based on actual locations and the name reflects that.