Yes, You Can Iterate Quickly In The App Store

You can change the responses from your API without recoding your app, if you design it correctly.

However, Apple will catch on to this eventually, so it won't be a free ride forever. They'll eventually want to approve API changes that your app accesses, and I'd bet they'd yank your app if you change the API (and thus the app) without their approval.

And if your app suddenly starts violating their terms because you changed your API, I'm sure they'll yank it.