Well, regarding Kermit being nice to Piggy... that's a part of his character. Kermit growls, snips, and snarks at the muppets around him... and often, hey, no wonder! They drive him CRAZY! But you know, deep down, he loves them, all of the insane members of his family, and always will, no matter how often they push him off the deep end.
But... it's not always easy to see that love... famial love, (I'll keep the gushyness out of this discussion) in Kermit's snarkiness towards Piggy. Certainly it's more fierce than towards any other muppet... and you can argue that Piggy drives him crazier than any of the others... but there are times when it goes beyond simple tension relief and jokes... it gets cruel.
I'm not saying Piggy can't handle it...I'm not even saying she doesn't deserve it. (I don't think she does, but I'm not focusing on that.) I'm saying it violates Kermit's character. He snips, snarks, grumbles and complains, but when push comes to shove, he doesn't mean it. He wouldn't deliberately hurt anyone.
Kermit's not mean. He just isn't. In a slew of five minute interviews, where Piggy's not around to snip back or fluster him... he comes off as one-sided and mean, at least he does sometimes anyway.
In a show... (boy, are MY fingers crossed), Kermit doesn't have to treat her like a china doll, or even like a girlfriend, to show that he cares about her... and he does. Kermit cares about all of his friends. I think that's all Ru is saying... just that it gets hard to watch him use her constantly as a target for cheap jokes. (There are hysterical jokes between them, at her expense, but... it just isn't funny when it's ...uncaring. Doesn't really make you like Kermit either.)
He IS a nice frog... to everyone... even when he is also a tired, grumpy, sarcastic, frustrated frog. Let Kermit be who he is, that's all.