People often laugh because they are surprised to hear crude words like "shit" coming from someone so small and delicate. The contrast between her tiny stature and the harsh language catches others off guard. Her quiet and sweet voice makes the blunt words even more unexpected.
The quote highlights how perceptions based on appearance can be misleading. It shows that even someone who appears gentle and unobtrusive can have a bold or expressive side, breaking stereotypes and challenging assumptions others might hold about them.