Back to Kermit's performance here...well, there's no making everybody happy. It's officially been 20 years of Steve as the frog and I adore his Kermit. Incidentally, there was substancial knuckle used in forming his face during this clip as noted in the famous Jim/Frank/Frith discussion. The "duck" claim must be coming from a different performance or in the funny faces pulled by the frog at the end of the segment. I'm ecstatic about Steve, Bill, Dave, Eric and the gang working on the new Muppet movie without need for recasts!
Not sure how much of an old-school Muppet fan you are, but there was a video that the three of them did, where Jim and Frank brought out their main characters and talked about how they perform them, where the voices come from, what the personality is of each, and so forth. It's an interesting interview. In it, Jim talks about how, when he performs Kermit, he keeps him just slightly angled forward, because if he kept Kermit at an upright, straight angle, he'd look like a duck. The expression on his face was halfway to his expression when he's mad and scrunches his face up. He says, "You wanna arch him towards the camera a little, otherwise you have Kermit the Duck."
If you want proof Steve does this, check out the May 14, 2010 issue of Entertainment Weekly on page 14. Steve does the duck face. I started really noticing it when they did those god-awful Pizza Hut commercials with Jessica Simpson... it's not just how Steve performs him, it's how he operates him as well.