Nintendo has published a great Iwata Asks interview with the folks behind Paper Mario: Sticker Star that is worth a read. Perhaps the most interesting part: Mario creator and legendary designer Shigeru Miyamoto actually convinced them that story isn't necessary to a Paper Mario game. Which might be why, when compared to the other three Paper Mario games, Sticker Star's story just isn't up to par.
Here's an excerpt:
Iwata: Miyamoto-san really persevered with Paper Mario this time. Exactly what was he particular about?
Tanabe: Aside from wanting us to change the atmosphere a lot, there were two main things that Miyamoto-san said from the start of the project-"It's fine without a story, so do we really need one?" and "As much as possible, complete it with only characters from the Super Mario world.
Iwata: That's a difficult task. In some ways that would be the exact opposite direction from recent games in the series.
Tanabe: Yeah. With regard to the story, we did a survey over the Super Paper Mario24 game in Club Nintendo25, and not even 1% said the story was interesting. A lot of people said that the Flip26 move for switching between the 3D and 2D dimensions was fun.
Iwata: That project came together because of that idea. Kudo-san, you were in charge of the script, so what did you think about whether a story was really necessary or not?
Kudo: I originally saw it in a way that's similar to Miyamoto-san. Personally I think all we need is to have an objective to win the boss battle at the end of the game. I didn't think we necessarily needed a lengthy story like in an RPG. Instead, we looked at the characteristics of a portable game that can be played little by little in small pieces and packed in lots of little episodes and ideas. I always did like putting in little ideas, so I actually enjoyed it.
Hmm. Hmmmmmmmmmm. I don't know if I like this. I don't know if I like this one bit. Maybe that's why Sticker Star wasn't my favourite.