Let's keep a couple of things in mind here.
Miyamoto is credited as Supervising director on Star Fox Zero. The only other game he's credited in the same way is Mario 64 DS. Seems like a minor role.
Miyamoto is not credited at all in Sticker Star. And the only thing we knew he did was deemphasize the story. But the story wasn't the problem, the problem was that the gameplay was completely broken.
Miyamoto has been credited as the designer of only 4 games in the last decade. Super Mario Galaxy, Steel Diver, Wii Music, and Wii Fit. So, one mega hit, one of the best reviewed games of all time, a fun diversion, and a piece of shit. Not a bad track record.
Miyamoto is credited as a producer or supervisor on... Smash Bors Wii U., Luigi's Mansion Dark Moon, Splatoon, Mario Kart 8, Skyward Sword, Mario 3D Land, 3D World, and others.
So, my conclusion is that it's hard to really tell if Miyamoto has lost his touch. He's involved in a huge deal of projects, and how involved he is in each is hard to say.
But, to say he's losing his touch based on a suggestion he made on a game that would be broken with or without his input (that he wasn't even involved with enough to be credited) and one other game is pretty darn stupid.
