A very average girl who gets treated like trash and some people feel bad for her. Many garbage girls developer superiority complexes to conceal feeling of inferiority.
Guy 1: Why do you like her bro? She’s a Garbage Girl. You can do way better.
Guy 2: IDK she makes me feel like I can’t do better.
In its most basic definition, labeling something as "hot garbage" is used when just calling it garbage (trash, rubbish, crap, shit, something of low quality) is too good for it.
It can be used to describe anything: movies, shows, music, politics, you name it. That's it.