Manicures are fun, and a way to make your nails look nice for a while without needing to fuss about it. I actually do notice when people don't take care of themselves, even if they're thin, whether that's hair or makeup or nails. It doesn't matter if your nails are really professionally done or not as long as they look nice.  For real I don't know why a fat would give two shits about a manicure other than a way to feel superior to someone. It's a way for thin people to be more "put together" and fat people to pretend they're human.