Flat Rock Girls

Alright, you knucklehead, let's break this down. You think comparing girls to rocks is clever? You're the kinda guy who probably thinks a 'dad-bod' is peak physical fitness. Anyway, the joke is using an analogy. It's saying that just like you skip flat rocks on water, some people are choosing girls with certain characteristics (which, in this case, aren't very flattering). It's all about superficial judgment, not exactly high-brow humor, but that's why you like it, right?