In fact, her best friend Shay drives a lot of the change in Tally. It is Shay who shows Tally how to sneak out of the city, who first questions the ethics of the operation that makes everyone look perfect. Shay starts off as a very strong and self-aware character, while Tally is still caught up in petty concerns.
I can see why Tally is the main character, however. She has room to grow. She transforms from normal teenager to crazy bad-ass by the end of the series. On the other hand, Shay starts off so strong that the only interesting thing to do is have her succumb to her flaws, which she does beautifully.
Tally's shallowness at the beginning might lead some to never finish the series or even the first book. That's the danger in writing a character that might come across as overly shallow or whiny or weak. Sure, as the author you know it's because you've planned a dramatic growth arc for your character. The trick of course is still to make the initial character likeable enough for the readers to engage with.
Personally, I thought Tally and the world itself was intriguing enough to keep reading, even as she was on the verge of betraying her friends. On the other hand, I've also read books where I could tell the author wanted to show growth in the character, but the MC started out so weak or TSTL (too stupid to live) that I could never get into it.
So which character did you like better? Tally or Shay? And which would you have written as your MC?