basically this is anyone who is considered a space case, twink, or ditz. It is usually referring to girls but it can apply to guys as well. the person must be in high school and must be considered to be one of the "popular" group. this is what the "pop" in poptart refers to. the "tart" part of it is easy: fake, slightly stupid, and annoying. Tart is an old insult so it makes sense here.
That girl is such a PopTart. Why dosn't she ever think for herself?