Green’s Dictionary of Slang

(US campus) a superficial young woman, pretty but lacking in intelligence.

1989		P. Munro Sl. U. 105: Heather superficial girl who’s a trendy dresser.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 ‘Heather’ is used for a girl who is very superficial. ‘Heather’ comes from a movie called Heathers which tells us that euphemisms, at least in university students’ language, change all the time.