The lips should be thinner on the stretched parts. Think of the mouth of a balloon or your own lips. When you open your mouth wide, the sides of your mouth stretch thin; when you stretch the mouth of a balloon the top and bottom where you're holding it are relatively thick and the sides are stretched very thin. Additionally, the more the mouth is stretched, the straighter the sides should be, as if they're being pulled taut. There should be no "thickness" lines at the sides of Fluttershy's mouth (the ones on the top and bottom are fine) and the sides should be straighter to convey stretching (top and bottom should retain the anatomical mouth shape).