From Muppet Wiki

Big-Fat-Ugly-Bug-Face-Baby-Eating O'Brien was a deep-voiced, yet beautiful pirate seen in Muppet Treasure Island. She is featured just briefly in the crew roll-call.

The character was part of the several name gags in the sequence (such as Old Tom, Real Old Tom, and Dead Tom). The nasty name of the beautiful woman was meant to comically parallel the beautiful name of the nasty monster Angel Marie, who was next on the roll call.

The character is never seen again in the film, neither on the ship nor the island – leaving the audience to wonder what happened to the beautiful Ms. O'Brien after roll was called.

In the DVD audio commentary for the film, Rizzo asks, "What was up with her?" to which Gonzo replies, "Don't ask, don't tell."