To me, that sketch is always funny. I got The Muppet Show Cast Album for my kids, which includes an audio-only version of that sketch. I've listened to it dozens of times. Always funny, for me, at least.
I must admit that I was surprised to learn, from the recent Jim Henson biography by Brian Jay Jones, that this sketch was written by Jerry Juhl right before it was filmed. Frank Oz and Jim Henson read it through once and then did the sketch in one take. According to Juhl, as quoted by Jones, it was the moment when Fozzie really came into his own.
John Seavy over at Fraggmented wrote about the Muppets as part of his storytelling engines series of blog posts. He characterizes the show's engine as the concept that failure is funny:
Looking at the Muppets, you see a group of people united by a) their passion for entertainment and their dram of making people happy through art, and b) their lack of talent at their chosen field. The gap between their desires and their actual abilities provides fertile ground for chaos, confusion, and comic misunderstandings as events slowly (and sometimes quickly) spin out of their control.
Seems like a good way to describe the gag behind that sketch. I love how enthusiastic Kermit is at the beginning, and how frustrated he is when he says "Look, the comedian's a bear" for the last time.