Before anybody starts jumping all over me assuming I'm just perpetuating the whole Steve Whitmire/Matt Vogel/Kermit fiasco, this honestly has nothing to do with any of that - this is just something that's been filtering through my mind for a little while.
Anyway, while it's true that Steve's Kermit voice was obviously very different from Jim's, one thing I think helped Steve's Kermit work character-wise was that like Jim, Steve is a "good ole boy" from the south. Of course, Kermit wasn't necessarily a southern-fried character or anything, both when he's not getting really flustered by all of the mayhem of the other Muppets around him, Kermit did have that sort of calm, easy-going charm about him that a lot of (non-redneck) southerns seem to possess . . . and, as Brian Jay Jones's book notes, an easy-going southern charm seemed to stick with Mississippi-born Jim throughout his life . . . that was a quality Georgia-born Steve was able to bring to Kermit as well, which is one reason why I think his Kermit still felt natural and worked well.