As far as I know, Betty Lou was one of those characters who for some reason was shown far more in storybooks and other merchandise than she ever was in the show itself. She seemed mostly to be confined to appearances in inserts here and there, and even then usually as a minor character in a crowd. (I like how these sorts of scenes used to speak volumes about how large SS's world was----a background crowd of Muppets might be comprised of one or two second or third-tier characters, like Betty Lou, and a whole bunch of random people we didn't know-----just like a crowd in real life!) She occasionally took the spotlight, such as that "Amigo, Amigo" song that showed up a lot in the 80s and 90s, but that seems to have been the exception. But in the mid 90s, there was an attempt to give her a more prominent role in Street stories, and she acquired a consistent performer for the first time (Lisa Buckley). But I don't think that lasted very long. (One such episode from 1996 can be seen at the Paley Center in NYC, in which she keeps excluding poor Telly from all her games because no boys are allowed.)
But if you read stuff like "The Sesame Street Dictionary," you'd be forgiven for assuming she was one of the main characters, since she's all over that book.