Oh, Penny. Where have you been all our lives? Off chasing moonbeams and polka dots with your other Manic Pixie Dream Girl sidekicks?
Manic Pixie Dream Girl is a term coined by critic Nathan Rabin to describe female leads from romantic comedies like Garden State and Elizabethtown. According to him, a Manic Pixie Dream Girl is a female character whose only purpose is to teach the sad, sensitive male lead how to enjoy life. She's characterized as youthful, uninhibited, and free-spirited—and usually has a wardrobe full of whimsical clothes and childlike hobbies.
In many ways, Penny fits right into the Manic Pixie mold. Think about how Judd says that "she could pass for a college student" when he sees her for the first time (16.1). He even describes her honesty as "gratuitous" (16.10).
However, it actually seems like Judd is aware of the concept of the Manic Pixie Dream Girl. Although he loves her honesty and youthfulness, he never quite falls in love with Penny as deeply as he might like. Similarly, we get glimpses into the darker side of Penny's life: her family problems, messy living conditions, and psychological issues. And we get the idea that being the Manic Pixie Dream Girl might be something of a crutch for her: she says that she has:
No great traumatic event to blame my small life on [...] I tried to make something of myself and I failed. That happens every day too. (16.31)
This moment undercuts the whole image of the MPDG as being free-spirited and uninhibited. Instead, Penny just … doesn't have that much going on. That's because the "Manic Pixie Dream Girl" doesn't exist in real life. Real people are complicated—they have good qualities and bad ones. To his credit, Judd has too much respect for Penny to trick himself into thinking otherwise.