Again, it's not a Muppet thing, it's not even just a Disney thing. Every company has to deal with stock photos and style guides because that's what the higher ups are looking for. Brand unity and easy recognition. In the case of animation (just throwing this into the conversation), the style guides rarely are identical to the show proper due to the fact they were taken from the earliest designs. Then the art evolution of the cartoon series changes the character looks, and you're stuck with toys and t-shirts with
sort of looking the same characters. Best example I can think of is the Looney Tunes Show. The overly stylized characters look less and less stylized as the show progressed.
I know there's some Sonic the Hedgehog DVD out there that uses one of Milton Knight's original model sheet Robotnik poses. I'd love to see more weird original artworks being used and less style guide crap. And the funny thing is, those Ncircle releases actually commission new art a good amount of the time. Some very off model and ignoring style guides.
As for the Muppets... I agree. We've got these great new posers that Disney has been starting to use, and they're inconsistent with it.
I wish I had a bigger picture of this, but I don't want to make one....
They have new Piggy, Kermit, and (I'm going to guess) Pepe poser cards, but they're
still using pepper shirt Gonzo, the MTM DVD cover, and that exact Bunsen and Beaker pose from the 90's. I long for the day when we have a new batch of Disney made poser pics to put on merchandising.