Why Pepe says "okay" at the end of sentences actually was mentioned by Bill Barretta in an interview. I think it's from Muppetzine, and Muppet Central used to have it transcribed (for a time Muppet Central transcribed a number of MuppetZine articles but for some reason they were eventually taken off the site). And in that interview Bill says that he had an aunt who talked that way, so Pepe was based on her. I think I've seen another interview or quote about it somewhere as well.
EDIT: A quote from Bill Barretta about it appears on page 22 of "Muppets From Space: The Making of Muppet Movie Magic": "A lot of Pepe comes from my wife's aunt. She was from Madrid, Spain, and before meeting her I had never heard someone say 'okay' at the end of a sentence. I always thought of it as more of a statement. I had never heard it as a question, the way she would use it. I just thought that was kind of interesting and funny, and that's what Pepe's delivery is based on."