About Kermit, it's not Matt's fault, nor a case of the public being too picky. Kermit IS the most difficult Muppet to perform, ever. Period.
It's not just the "voice", guys, it's everything -- not only vocal pitch and words being said, but pauses and gestures, sighs and stops, subtle inflections and expressions, the way character moves when he has something on his mind, the way he fidgets when he doesn't, and a myriad of other ways we all express ourselves, something we always recognize in people we know well.
Bill Barretta once said, on performing his own and other people's characters (bold emphasis is mine):
"Yes there is a difference between performing characters that I've created or collaborated on, versus characters that someone else created first. I’m not really a voice match artist, so when I'm performing Jim's characters, they are fair impersonations at best. What I try to do is hopefully present to the audience a certain feeling or essence of the original character to help compensate for that imperfect voice match. After all, the voice is really only ten percent of the character for the Muppets. It’s the other ninety percent or the characteristics and personality of the character that makes them who they are and their uniqueness. I will never be able to actually become Jim's characters, because they weren't born from me. So yes, I need to be more conscious, rehearsed and thoughtful about the choices that I make for those characters. As opposed to performing my own characters which are a part of me, and so I feel more comfortable being able to live more in the moment with them."
— Bill Barretta, MuppetsHenson exclusive, Oct 18, 2021
Out of all the Muppet characters, Kermit may be by far the most difficult to emulate. Being the most rounded, full-fledged personality given to him by Jim, he is not easy (impossible?) to summarize in a brief character description (like "
Marvin Suggs: soft voice, hard mullets"). The "
good hearted, compassionate leader of the muppets" does not even come close to describing who Kermit is. I doubt if it can be expressed other than "that feeling of Jim's Kermit". (If you watch Jim Henson: An Idea Man, he actually says that Kermit is a version of himself.) It's a LOT harder to get and express that "feeling or essence" Bill was talking about for Kermit than for, say, Sal, Bunsen, Rizzo, or even Rowlf.
Moreover, a good performance involves lots of choices for every minute movement, change of pitch, breath and cough, and lots of other things a person does not even think about, consciously. Most of it is coming "from inside", decisions are made on subconscious level based on what performer has "in him" for the character -- most of it just comes naturally, some is painstakingly rehearsed, some gets absorbed from experiencing other people -- pretty much all of it constitutes part of the unique personality of the performer, ingrained in his/her/their subconscious, in their "soul". There are a number of quotes by Muppet performers that their characters often surprise them by saying things they did not expect -- the whole character performance process is really heavily subconscious-related.
Sometimes the other person has a compatible personality, similar emotional upbringing and experiences, the "kindred soul", or a similar part of their psyche (e.g. some people are naturally Kermit-y without trying to seem so). That helps them to pick up the character's traits and mannerisms, but even then it is not an easy feat -- it still requires a lot of concentration, discipline, talent, and professionalism to pull thru.
"Every time a character is taken over by a new person, it sort of jumps to another track. It’s a little bit different. Even when the new person has uncanny ability, it can’t be the same, because these characters come from our souls."
— Dave Goelz, Vulture.com interview, Mar 19, 2018
Matt does it perfectly with Jerry Nelson's characters -- I am 200% behind his Sgt. Floyd, and I am sure that Matt is the absolute best that could ever happen to Uncle Deadly. (Come to think of it, it might have something to do with the fact that Jerry was an Okie, and Matt was born and raised in Kansas City.) I have no doubts whatsoever about Matt's talent or abilities -- his own Constantine will always keep a special place in my heart.
However, with Kermit, Steve had a lot of advantage - he worked side by side with Jim for ages, they really liked each other (per Frank Oz interview), and he was given that useful advice "don't try to play Kermit. Play Jim playing Kermit instead" (alas, I lost the source for this quote). Even with that, he had a hard time accepting Kermit until something from his subconscious, in a dream, told him it's time to do so.
Despite what Tough Pigs are trying to instill in the public mind lately, there was no general community outcry against "Steve's Kermit" -- rather, based on my own experience and words of everybody I asked about it, people were pleasantly surprised that Kermit was still pretty much himself, and there was no "hey, wrong Kermit" jolt for MCC. (Thanks, Steve, really -- Thank You So Much for that, man!)
Matt, or any new Kermit performer would be at disadvantage for exactly the same reasons -- no direct experience of working with Jim, or insight into Jim's mind and Jim's upbringing (unless it's a fellow Southerner) -- and apparently Matt was told by members of Henson family when he was coached for Kermit's role not to take in anything from Steve, but "return to the roots" instead, which he tries to emulate from old recordings.
Alas -- When I went to see MCC I was ready to call it quits on Muppets -- and was amazed that I could believe the Kermit I used to know is right there on screen. When I see Kermit's performances past 2016, I really want everything to be up and running, but the only thing on my mind is -- no, not there yet. (Despite Tough Pigs desperate pleas "it's been 10 years now, shut up everybody". Even if these guys are not directly on Disney's payroll, they are privy to so many internal happenings there that they are probably not allowed to form their own opinions.)
Maybe somebody should give Matt advice like "don't try to play Kermit, play Constantine playing Kermit instead" and see if it makes things better?