The narrator's sarcastic and humorous observations contribute to the tone of the book. Melinda has just experienced something traumatic, and that experience—paired with her decision not to speak about it—separates her from others that she used to be close to, including family and friends. She feels isolated and alone and as if others don't understand her or her depression. Her humor and sarcasm are a way of coping with her difficult situation, and as someone who views herself as an "outcast," they also reflect her feelings of disconnection from others due to depression and social isolation.
One specific example of Melinda's dry humor is particularly revealing of her attitudes toward the significant people in her life. She has a way of assigning nicknames to those around her that indicate her feelings for them. She calls an aggressive and macho social studies teacher "Mr. Neck" and a mousy, scattered English teacher "hairwoman." The only class she likes is art. It becomes a safe haven for her where, as a selective mute, she can express herself and find catharsis through nonverbal pathways. Tellingly, she does not give the art teacher a sarcastic nickname. She calls him by his real name, Mr. Freeman, which already reflects her feelings toward him, since art is a space where she feels "free." It is also more personal, of course, to call someone by their real name, which reflects Melinda's feelings of connectedness in her art class compared to the other parts of her school day.