Once you get used to the fact that kids are called "littlies," it's pretty smooth reading. Actually, "littlies" is Australian slang for "little kids"—so readers from Australia have a head start here. But most of the rest of the slang sounds pretty much like stuff that could be said now: "bubbly" = good, "bogus" = bad, etc. So even the "new" slang words that Westerfeld uses are pretty clear. (What's a hoverboard? It's like a skateboard that hovers. Ta-da.)
Once you get used to a few new words, most of the sentences are straightforward, like when Tally thinks about her parents' advice to tattle on Shay:
And she had made Shay a solemn promise. Even if she was just an ugly, a promise was a promise. "Guys, I'm going to have to think about this." (14.52)
What's so nice about those sentences is that Westerfeld is talking about big, big, big issues: what Tally owes to her friend Shay vs. what she owes to herself (since she wants to be pretty and she won't be unless she breaks her promise to Shay). But the sentences are pretty easy to read by themselves. And these sentences build on each other: Tally made a promise; a promise is a promise; so she has to think seriously before she breaks that promise.
So reading the sentences isn't so hard here; but these easier sentences built up to talk about big issues. And once you start thinking about the issues, your brain might start hurting—in a good way.