Claudia is often compared to a doll for obvious reasons: she's petite, she's pretty, and she wears nice dresses. Lestat calls her a doll. Even Louis calls her a "magic doll" (1.433).
But there's an ickier reason they call her a doll: the latent sexuality within her. She's like a toddler with a tiara. There is something creepy about a young girl dressing up like an adult woman. Being called a doll diminishes Claudia; it reduces her to an object. Lestat is described as "play[ing] with her as if she were a magnificent doll" (1.426). That's a problem. Lestat manipulates her as if she were an object and not a person.
As if all this doll imagery weren't horrific enough, Louis ends up having a nightmare about it one point: "Can you picture it, this splendid domesticity, dim lamps, the vampire father singing to the vampire daughter? Only the doll had a human face, only the doll" (2.205). This nightmare is a glimpse into Louis's humanity crisis. He sees the dolls as more human than the vampires.
In other ways, all people in the novel are like dolls. Nobody's life has any value to these vampires. Everyone is an object—an object for sex, and an object for food. It's not a pretty picture. No wonder Louis is bothered by it.