I've been trying to refrain from commenting on the Miyamoto topic out of respect to other users because I know it can be annoying to people, but it keeps getting brought up again and again so I guess people are okay with talking about it.
I never even understood where all the Miyamoto hate comes from concerning Mario RPGs. Paper Jam had nothing to do with him, and for all we know, neither did Color Splash. What happened with Sticker Star was Miyamoto's remedy to Super Paper Mario's failure to acknowledge its Mario heritage, and while maybe that was a tad overboard, it was also entirely understandable.
Paper Jam is up to speculation, but to clear things up there is indeed a factual
source where Tanabe says Miyamoto told him and the other developers not to include original characters in Color Splash as well.
Does it mean Miyamoto is responsible for the trend of original characters and story getting nerfed? Not necessarily. If anyone else reading this is familiar with me, by now you might be thinking I'm about to unload on Miyamoto, but nowdays I really don't think it's helpful anymore.
To answer your question as to why he gets blamed a lot: I think the reason is because he's seen as the catalyst with Sticker Star, the one who got the ball rolling with the direction of no original characters and less story, even if he wasn't responsible for the change in each game.
I can see both sides.
A: It's Miyamoto's fault. He told
Sticker Star and
Color Splash not to have original characters and less story.
B: It's not Miyamoto's fault. He didn't tell Paper Jam, MLSS+BM or MLBIS+BJJ to remove original characters. Also the other developers could have incorporated his instructions better.
Both are true, I think both sides have fair points and it would be nice if both sides could respect how each other came to their conclusions even if they disagree with said conclusions.
Over the years, I've come to realize that talking about this with focus and speculation on Miyamoto isn't helpful and goes nowhere. The main issue is the direction the series has taken, regardless of pointing fingers at who's to blame. It's more constructive to focus on the issue itself rather than on Miyamoto.
It's totally justified for fans to be upset that Paper Mario (perhaps Mario & Luigi to a lesser extent) had original characters removed and story nerfed, those elements were a core part of the charm to many people. When a series you love removes a feature that made it enjoyable to you: it's normal to feel sad, disappointed, upset and rant about it or criticize it.