Hi,
What are your goals in terms of obfuscating your data structure?
If your application is returning data in JSON documents, presumably that application would also have the logic to de-obfuscate the JSON so it is usable. If you are aiming to protect IP, obfuscation of the data structure isn't providing any real protection from a slightly motivated user who wants to decode your documents. I think you'd really want encryption or some sort of digital signing to prevent source documents from being used without permission.
Document obfuscation isn't a MongoDB server feature, but you could perhaps implement some protection in your own application/API.
Stack Overflow questions are better received if you can include code examples for what you've tried. The Software Engineering Stack Exchange has a smaller audience but might be more receptive to a question about how to approach a solution (as long as you can describe what success looks like).
Regards,
Stennie