I've never heard of Vicky Lawrence or Thelma Harper.
After Googling some images, I'm torn on the likeness. As a caricature of Vicky Lawrence, it doesn't seem to follow her face shape (from the images I've seen), but it looks as though she pulled a certain expression as Thelma Harper which did elongate the chin and make it appear more pointed than her face normally was. So, it could work as a caricature of the character rather than the actress.
I can't really say any more than that, as I'm only going by a couple of photos.