Symbolism, Imagery, Allegory
It's bad enough having to move in with Aunt Casey and Uncle Toole, but having to sleep in the room where Casey keeps all her faceless wig model heads just makes the situation way worse for Miracle. Like, thanks but no thanks for the creepy, faceless company.
During her time living with Aunt Casey before her hospital stay, Miracle makes a habit of talking to the heads, but the conversations aren't exactly encouraging. In her mind, the heads are really a bunch of plastic bullies, as she personifies the heads by turning them into her worst critics. At one point, they taunt her for her love magician act at school:
They were waiting for me, lined up on the shelf, laughing their nasty laugh. "What do you know about love?" (15.2)
We all know plastic wig stand things can't actually laugh, but in Miracle's mind, the wig heads represent all the discouraging forces and thoughts in her life that keep her from seeing herself as a real, living person. And since they're basically blank slates, there's really nothing stopping her from projecting all her human anxieties onto their empty and waiting human forms. In these anxieties, then, we get glimpses of Miracle's worst suspicions about herself.
Need more evidence? Check out the conversation Miracle has with Dr. DeAngelis about the wig heads. She relates a number of characteristics of the wig heads that make them sound pretty sinister—she says they "watched me" (24.46), "They didn't have any faces" (24.51), and "They were dead" (24.73). Yikes. Miracle's view of a bunch of seemingly innocent plastic heads gives us a startling look at how she really sees herself: as faceless, dead, and constantly scrutinized and criticized by people around her. No wonder staying in that room gives Miracle the willies.