Remix routes have the ability to handle both backend code and UI code in the
same file. Remix loaders and actions are backend code that's tightly coupled
to the UI code for that route.
Additionally, you can define routes that don't have any UI at all. These are called resource routes. This allows you to create REST endpoints or a GraphQL endpoint to make your app data and logic consumable by third parties or additional clients (like a mobile app). You can also use this to generate PDFs, images, stream multi-media and more.
The Epic Stack has a few resource routes in place for managing images, the cache, and even has a few "full stack components" for components that manage the connection with their associated backend code. Watch the talk.
So, yes, you can absolutely use the Epic Stack to build APIs for consumption by third party clients.