Everything already stated is correct, but thought i'd tie it all together in more of a context.
Betty Lou was mostly done by Frank Oz at first, she wasn't even originally a "character" so much as one variation of the Anything Muppets - and she was pretty much the standard design for a "little girl". Eventually this was used so much that the design got a name - though still not so much of a personality/hook beyond "little girl". For several years after, she was played by any number of performers, often Fran Brill (sometimes Marilyn Sokol if for a musical number) but since Sesame's other "little girl", Prairie Dawn had more of an established personality and a regular performer in Fran, Betty Lou was more window dressing and good for merchandise since the balance between male/female Muppets was pretty grim.
Finally in the '90's there was more a serious effort to help correct decades of female imbalance with new characters like Zoe and Rosita. Betty Lou was given a regular performer with Lisa Buckley and some unique traits to help make her more of an actual character - her love of dolls for instance. But then as Buckley's involvement with Sesame eventually waned, so did the character's presence.