Rubberface.

A teen-age girl conceals her insecurity behind a barrage of jokes — until she meets an aspiring comedian who asks for her help with his new act. Janet is an over-weight girl who has a knack for making the other children in school laugh... by making fun of her own weight. In seeing the other kids' reactions, she feels that she might have what it takes to be a comedian. She visits the local comedy club where she finds Tony Moroni who is a struggling comedian whose jokes are less than funny. Together Tony helps Janet find self-esteem and Janet helps Tony with his material.