This is an interesting topic. I don't think I would really put this change on Steve. Nope. When I think about it, I agree with myself and don't. I agree with Ruahnna here and I'd like to suggest that perhaps the reason Kermit seems nicer, not that he was ever deliberately mean (in my opinion) without at least a little bit of cause, well, perhaps it has to do with the movies.
Just about all of the recent, post- Jim Henson years movies have been centered around the Muppets making movies... telling other people's stories and not really their own. I think if you ask around, a lot of MC'ers are wistful for the days when they were telling their very own stories, but I digress.
Movies are going to be different from the shows anyway, because in a movie, you aren't seeing the day to day hassles of keeping their mad little family afloat, you're seeing a specific set of events pertaining, more or less, to the plot. A movie is more focused on its central theme than a show is, and as such, we lose a lot of those lovely, meandering little character moments that a lot of us, (well, I suppose I can only really speak for myself here) really enjoy.
To further the effect of not seeing as much of Kermit's day to day character, he hasn't been playing himself lately, he's been, well, acting. The exception to this, and here I leap over Muppets From Space because, though he was playing (or being) himself, he wasn't really central in it at all, the exception is It's a Very Merry Muppet Christmas Movie (oy, what a title!). Kermit was central again, and playing (or being) himself... and what did we see? A stressed out, occasionally arm-wavingly hysterical frog who snapped at his close friends when it all got to be too much for him. He was still Kermit however, still loving, still kind, still well-meaning to the very core of his being... just worn out from the day to day burden of being everyone's fearless leader.
I do realise of course, that there are many deliberate parallels with IAVMMC and It's a Wonderful Life, rather than a truly original story but it was still the Muppets being themselves again, in a situation that they've surely found themselves in more than once. (Go read Lisa's Summer in the Theater!)
Back to the point:
Is Kermit a jerk? Well, he's certainly got his moment of crankiness, grumpiness, sarcasm and stress, but don't we all? If I snap at a family member when I'm frustrated over something, does that mean I love them less? That I'm a jerk... all the time? As part of my personality? I don't think so, and I don't think so for Kermit. Does anyone doubt that he genuinely cares about each and every one of his friends? Or that he would do anything, anything at all possible, whatever it cost him, to take care of them? Kermit has his moments... but perhaps it's better to assess his character based on all of his moments, not simply the ones where he's holding a show together with spit and wishes. Let's not forget that he's working for all their dreams... and goodness knows they don't make it easy sometimes!
What he needs to do, I suppose, is learn to rely a bit more on that mixed-up, but lovingly devoted family he surrounded himself with, so that he doesn't feel like he has to bear that burden alone. And he should relax enough to let Piggy love him the way she wants to, and deep inside, he wants her to... but again I digress into an area I'm
extremely biased in!
<slinks off back into the fanfic section, where she belongs>