Xibalba is the name of the mayan Underworld.
In the spanish dubbing of the movie, La Muerte is called "La Catrina." This is obviously "Posada's Catrina," a popular iconic skeletal lady that has become associated with the festivities of the Día de Muertos. Her main attribute is a gorgeous hat. In the movie, the modern Mexican icon (La Catrina) and the classic icon (Lady Death), are fused in a single role as the ancient Mayan goddess of death, ruling over one of the lands of the dead.
As revealed by director Jorge R. Gutiérrez on Twitter, in character posters for María, her arms and hands are depicted more flesh-like, as opposed to having disjointed pieces like in the actual film. This is because the studio felt audiences would be turned away by a movie about living dolls. When Guitierrez brought up Histoire de jouets (1995), he was met with blank stares.
The original songs "I Love You Too Much" and "The Apology Song" were co-written by Paul Williams. According to director Jorge R. Gutiérrez, when he approached Williams, the composer assumed it was because he had co-written the Oscar-nominated "Rainbow Connection" for The Muppet Movie (1979) and was surprised to find that it was because of Gutierrez's love for Williams' rock musical Le fantôme du paradis (1974).