The mudfrogs metaphor is a literary device that is used to promote one of the main themes of this book, which is the conflict between individuality and conformity. This is something that occurs throughout the book and finds its focus in the way that Stargirl represents the forces of individuality and the High School where she attends represents conformity. This conformity is so strong that the students represent mudfrogs in the way that they are all together making exactly the same sound and doing exactly the same thing. None of them dare to be different or to try and express who they are.
Stargirl is of course the antithesis of this. She not only expresses her individuality through her dress, actions, and behaviour, but also makes it clear that she doesn't care about whether her fellow students approve or disapprove of what she does. She is a character who rejects conformity and sees it as something that is restrictive and negative, as her experience when she tries to conform amply demonstrates. The mudfrogs metaphor therefore helps emphasise the central theme of this story.