To picture the end of Uglies, just imagine a giant jar marked "IRONY" falling off a shelf marked "CLIFFHANGER."
Pretty Shay and ugly Tally go to turn themselves in to the government of the city that they spent most of the book fighting, which is a little ironic. More ironic is that Tally spent a large part of the book wanting to be pretty; but now, at the end she doesn't want to be pretty anymore. And now she's going to get the chance to be pretty when she no longer wants it. That's irony with a capital Alanis Morrissette. (Note: not all the examples in that song are really irony.)
This might seem like a sad, tragic end—the big guy wins, the underdog loses. Usually, we like it the other way around, where the big guy loses and the underdog gets carried off by people in a parade. But that doesn't happen here. Instead, we basically get a big neon sign flashing "to be continued." We know there's more to come because Tally has a whole plan about being turned pretty, then kidnapped back, then given the brain lesion cure, and then something else. At least, that's her plan—and we'll have to read the sequel to see how it all turns out.
Some readers might not like this cliffhanger ending because it's so, well, cliffhangery.But we're suckers for irony, so we like it. And we dig that last line:
"I'm Tally Youngblood," she said. "Make me pretty." (50.49)