I think that's another example of what's already been discussed in this thread before: often times whenever a new performer takes on an already-established character, they start out as nothing more than imitations of the previous performer, before they start making the characters their own. Marty's first couple of seasons as Telly he tries to give him that same kind of neurotic twang that Brian had, and likewise, he tried to give Snuffy the same kind of soft, emotionless voice that Jerry had. Over the years, both characters took on so much more life and emotional range that the characters became dynamite - even Joey Mazzarino can't figure out why kids don't like Telly more than they do, despite all the writing they've done for him.
Getting back on topic, here's another reason why Kermit's "bitterness" and "depression" can't all be blamed on Steve: many writers are notorious for not letting actors and performers change or deviate away from the script, and said actors and performers can and will attest to that. Even in cartoons; Pat Fraley and James "Uncle Phil" Avery have said in the first couple of seasons of the 80s Ninja Turtles cartoon, the voices were given a little more freedom to ad-lib during recording sessions, but afterwards when the series became a solid hit, suddenly the writers got very strict in making sure they all stuck to the script, up to a point the voice actors would beg to get in a least one ad-lib from time to time. A lot of single camera shows (as THE MUPPETS (2015) was) are like this too; multi-cam shows in front of audiences are a little less strict about it, much like the 70s version of THE ODD COUPLE, where Tony Randall and Jack Klugman's performers were very similar to Jim and Frank as Ernie and Bert, in that they would often do the scenes and play off of each other in their own way. So again, if Kermit has come across as "too bitter," and "too depressed," I think most of that blame has to go to the writers - because again, Disney won't bring in actual Muppet writers, who could have probably written Kermit differently.