I wonder if Jim Henson had ever considered making Lips become a major character. In Steve Whitmire's Muppet Central interview, he says that he wanted Lips to have a Louis Armstrong-typ voice, but was worried about offending african americans. I have seen two interviews with Dave Goelz where he says that whenever Zoot was given lines in the scripts that he would try to give the lines to other characters whenever possible. I wodner if Steve Whitmire gave any of Lips' lines to other characters. Zoot did have quite a bit of dialogue in The Great Muppet Caper, the only movie where Lips was part of the main cast. And some of Zoots lines seemd unneccessary. Him telling Kermit that Animal was upset over missing a rembradt museum could have been given to Floyd or Janice. After Janice hears that Kermit would be dating Lady Holiday and says "Wait 'till I tell the guys in the band!", Dr. Teeth, Floyd, Lips, and Zoot all appear, and Lips is the only one of them to not talk. And Zoots line, "What's goin' down", isn't really a very neccessary line.
When the cast picks up Kermit and Piggy for their date, Lips has a few off-camera lines. I wonder if this was a deliberate choice. Of course, we know it's him, since that's a Steve Whitmire voice and no other Steve Whitmire characters were there (unless the voice was meant to be that of a chicken). In that scene the voice sounded like a mix of Floyd and Zoot, so casual fans might have thought it was one of them.
It also seems like Lips was absent from a lot of key scenes in the movie. Especially ones that were photographed and included in many books r promotional material. He was the only member of The Electric Mayhem to not ride a bicycle (a full-body Lips existed in the Rockin' Robin number, so it's not like he didn't have legs). He also wasn't in the Muppets room when Gonzo told everybody about the jewel thieves plans (but Rizzo wasn't there either). And he wasn't in the scene where the Mupepts read off the checklist. And when the Muppets get to the mallory gallery, it seems like he keeps being part of the group and being nowhere to be seen.
It seems like Lips was also rarely illustrated. I know that Lips was in The Muppet Show Book, but was he in Muppet Magazine or any of the Muppet annuals? I don't know if he's just too hard for proffessional illustrators to include in books, but many other obscure characters have been illustrated in books. He wasn't even included in any of the Great Muppet Caper glasses from McDonalds, not even the one with the Happiness Hotel bus (even Sam was included in that glass, and he never rode the bus).