To put a pin on the point, I think the main problem with the Paper Mario series is consistency. It was born as an experimental RPG on the Nintendo64, which matured exponentially on TTYD with a battle system that had LOADS to offer (every battle takes place in a theater stage) and a compelling, well-planned story.
Then, the Paper Mario series entered a weird "puberty" phase, where it lost it's very essence and reduced it to paper gimmicks.
Super Paper Mario was a nice and polished game, but it lacked the flow of the previous ones as it was a platformer whose RPG elements proved to be unnecessary.
Sticker Star was an attempt to go back to form, but as the interview states, that attitude of "leave RPG stuff to Mario&Luigi" left Paper Mario with a loss of entity: "If M&L uses proper battle mechanics and character levelling, what do I have?" The result is the much-loathed battle system of Sticker Star, with bosses that lack strategy and guidance for the player on how to progress. And don't make me talk about the bigger stickers (the fan, the cat statue, etc), all flash and no substance.
(M&L Paper Jam doesn't count since it's a filler game)
Now Color Splash features almost literally the same stuff fans hated on Sticker Star, but with prettier graphics. Well, let Nintendo learn from this one, if it can.
Paper Mario was born to be a quirky, well-produced RPG franchise, but all this flashy BS they like to throw at it only hurts its reputation down the line.