I would categorise it with the first three games for several other reasons as well, story, sound, characters, etc. Paper Mario went through a big change with Sticker Stars.
You're right in that it's not an actual turn based RPG. But it never fully left its RPG roots with the way it incorporated leveling up and other mechanics in the game.
And neither did SS, CS, or TOK. Paper Mario never really left its RPG roots, people just criticise the latter games because the battles feel pointless. And I agree with that sentiment. I loved the overworld gameplay in CS and TOK, but the battles hold those games back for me. So I personally hope they focus on fixing that aspect of Paper Mario. As well as having a basic Peach is kidnapped story, that should have been over and done with after Super.