I'm sorry, but from everything I've read and seen...Stoller and Bobin don't 'understand' these characters at all.
I'd prefer not to rant about the whole matter, but I will touch upon one aspect.
http://www.toofab.com/2014/01/07/muppets-most-wanted-set-visit-secrets
Apparently, Tina Fey (a human, if I recall correctly) is getting a musical number, "The Big House". While of course Gonzo only gets to sing a few verses in the whole movie (I don't recall he even sang at all in the last movie).
I at first wished to think of it as, it's no different than, say, the celebrity numbers on the Muppet Show. Not every number was Muppet-focused, and we accepted that just fine.
But, I'm afraid there is a difference. According to the autobio, Jim wanted to make sure he was on good terms with every guest on the show. So there's kinda this need to make sure the creative staff was on the same wavelength as the guest. That hasn't happened here. It's pretty much Hollywood people hanging out with other Hollywood people.