If you mean a 'tween-aged reader and not a twin, then yes, I'd say it is appropriate in the sense there's no profanity, violence or explicit sexual content. However, I can't speak to what you mean by appropriate in the cultural sense. If you're a devout Christian, for example, you could find some of the inferences Fey makes objectionable.
My parents would have let me read the book at a young age and its influence wouldn't have been damaging to me; I'd let my kid read it as well. That's all I can really tell you. Personally, I'd be more worried about what they're watching and reading on the internet, where there are fewer constraints and people are far less euphemistic.