This is an old trope (I've heard it on occasion since the 1950's), and it's no surprise that MTG relies on old tropes (rather than truth and logic) for her wisdom.

The stereotype goes something like this: Black people are worse (in whatever way---presumably all ways) than White people, but White trash are worse than Black people.

The presumption is that since White people are inherently superior, White trash people are betraying their own kind by being inferior.

Of course one key motivation for racism is to feel superior when you know (at some level) that you're not truly any better. So Greene's trope allows her to feel superior to two groups at once.

I'm in Greene's Congressional district. People love her here. You should be at least as worried about her supporters as about her.