Tally Youngblood always wanted to be pretty. She believed that the operation was a good idea because it would make everyone equal. She spent 4 years of her life looking longingly out her bedroom window, wanting to be in New Pretty Town.
Now, Tally sees that being pretty only segregates people more. Lesions included in the operation make all pretties act the same. Tally was too late to save Shay from these lesions.
If Tally didn't change her thinking by the end of the book, she could have ended up in New Pretty Town, partying and not caring about anything for the rest of her life.