Having explored a lot of app bundles this way, I can report that many iOS apps have shaders and other similar files (e.g., plists, HTML files) stored as plain text in the bundle.

PNG files are obfuscated, but that's reversible:

http://www.axelbrz.com.ar/?mod=iphone-png-images-normalizer

This kind of spelunking can be a good way to learn from other developers about how to approach some problems. However, I agree with other commenters that it would be morally shaky to re-use any of another app's files in my own app.