Hello, I am not sure that you are still following this post, but I just find a way to apply Sky Atmosphere on Mobile.
Take a look at Sjoerd De Jong tutorial about how to use Sky Atmosphere: https://youtu.be/SeNM9zBPLCA?t=560 (Material integration)
I followed the same steps except adding cloud texture in the video and put the material in a sky sphere (the sphere’s static mesh needs to be SM_SkySphere) then I can see the sky atmosphere in the Mobile Rendering pipeline (both Android ES3.1 and iOS)!!!
Make sure you make the sky sphere large enough so you can see the space view as well.
You can follow Sjoerd’s tutorial from the beginning to the end of the Material Integration section, and you would see the sky atmosphere works on Mobile.