You're right. Kermit isn't the same as he was when Jim performed him. But have you ever compared early Sesame Street Kermit with The Muppet Show Kermit? Or The Muppet Show Kermit with The Jim Henson Hour Kermit? The character changed a lot between SS and TMS.
That's actually an excellent point. Kermit evolved quite a bit from Hey, Cinderella to The Jim Henson Hour but you know what separates Jim from Steve? Well first and foremost, it's Jim's character and personality. But secondly, Jim actually improved. While Jim wasn't always perfect, there weren't any painful moments by him that I can remember. It's actually tough watching Steve do Kermit for me personally. It's embarrassing. And it's embarrassing because, here's this iconic puppet frog that had more character, personality and depth than most fictional characters; someone who could resonate with anyone at any age, and after the performer passes away, he's handed to someone who brings him down several pegs to where it seems only children find him truly appealing. It's even the little things that he's done with Kermit, like keeping the hands positioned way too high or scrunching the face up too much.
The worst of it is, he turned Kermit into this, "Hey everyone let's all get along, sing, and listen to me talk about either Miss Piggy or how I'm just a little ol' frog. And I'll do it all while seeming foppish, air-headed and one-dimensional ." It's intolerable, and Piggy's nearly the same way now. Kermit is now pretty much like the hot yet dim-witted girl you'd meet at a bar. Looks appealing at first, but when either of them start talking, you see there's no substance.