why is coraline considered to be a postmodernist novel?

This has the scent of homework.

I don't know that I'd consider it particularly postmodern. It's a fairly clearly written "young girl triumphs over supernatural threat that's too big for her" sort of story.

well, unless you go with the idea that we live in a postmodern era, and that therefore every work of art that is created in this era is postmodern by default.

that said, coraline doesn't really have any of the traits that i would consider postmodern (pastiche, quoting "low culture," self-awareness that it is a story and/or commenting on it, etc.)

I take postmodernism to mean that there is no truth, only what is true for the individual. (ie no universal truth that is true for everyone)