We kind of feel sorry for Peris, because we bet he used to be cool. When he and Tally were both uglies, they used to play all sorts of ugly tricks, like breaking into New Pretty Town and basically causing a fuss. So Peris was probably like Tally and Shay: a risk-taking trickster.
That's why it's so depressing to finally meet Peris: he's just another pretty going to get drunk at a party and worried that he's not wearing the appropriate clothes: "He blinked his long lashes, regarding her masked face, then looked down at his own tailcoat. 'Oh, dear. Wasn't this party white tie?'" (2.38).
So, if you look at this one way, Peris used to be cool and independent, and now he's just a regular pretty, worried about what he's wearing. In a word, he's changed. As Tally notes, Peris used to make fun of the shallow new pretties, "but he hadn't waited a moment to join in the fun" (32.26). Sure, Tally should be more sympathetic—she thinks this after she knows that there's a pretty good reason for the goony way he's acting. You know, brain damage. But whatever the reason, it's clear that Peris has gotten boring and ordinary.
So that's why we say that Peris is a cautionary figure by the end of the book: he's an example of how the pretty surgery can make someone less interesting and independent.
Peris has another important role in the first half of the book: he's the reason Tally keeps getting into adventures. At first, all she wants is to see and hang out with Peris. That's why she breaks into New Pretty Town, which leads to her meeting Shay. And when Special Circumstances tries to blackmail Tally, she only agrees to help them because Peris asks her to, so they can be pretty together—which leads to her quest for the Smoke.
About halfway through the book, Peris no longer seems to be super important to Tally. In fact, Peris seems a lot less interesting than oh, say, David. When Maddy and Az tell Tally about the brain damage, Tally understands what they mean because she can compare Peris (brain damaged) with David (excitingly not brain damaged) (32.13).
And that's why we feel sorry for Peris.