How we cite our quotes:
There was a certain kind of beauty, a prettiness that everyone could see. Big eyes and full lips like a kid's; smooth, clear skin; symmetrical features; and a thousand other little clues. Somewhere in the backs of their minds, people were always looking for these markers. No one could help seeing them, no matter how they were brought up. A million years of evolution had made it part of the human brain. (2.47)
This is at the heart of Tally's understanding of prettiness: it's not something arbitrary and just made-up. It's part of everyone's biology that says "big eyes" = "I want." That kind of makes "appearance" sound like some all-controlling tyrant—"oh, I hate that person, but they have big eyes, so I want." It almost seems like there's not a lot of freedom in Tally's version of the world.
"Shay! Come on. It's just for fun."
"Making ourselves feel ugly is not fun." (5.60-1)
Here's the downside of prettiness. (Also the downside of photoshopping people's bodies.) If everyone agrees that "big eyes" = "I want," then people with small eyes will feel bad about themselves. (And possibly have trouble seeing.) Is it possible to imagine a world where everyone got surgery at 16 to be "pretty" and where everyone under 16 felt okay about how they looked? (Probably not.)
Shay splashed a handful of water at her. "You don't believe all that crap, do you—that there's only one way to look, and everyone's programmed to agree on it?" (10.58)
Tally would say yes to this—she thinks there's one way to be pretty and we all agree because of biology. By contrast, Shay doesn't believe any of that, and thinks that people have some freedom to choose who they think is pretty. (To slightly support Shay, here's a bunch of ladies from famous artworks photoshopped to make them look more like what we today think as pretty. (Parental advisory: they're nude.)