It's the same as saying "American as a burger", arepa is just a traditional food in Colombia, a bread made of corn

Ok but I was wondering if you'd describe the "look".

this should help https://www.tiktok.com/discover/Paisa-girls

to be honest, I don't actually know but I've heard it from multiple people when I was in Medellin in reference to girls. none of them were "flat and round." I think it just refers to them being from the same area. "american as apple pie"

Exactly. It's this.