I made a little comparison of the upload of a Tilt Brush scene to Poly and to Sketchfab.The good news...with some work tweaking the material settings and lights, it could look quite good. It took me only a half hour to get the start scene optimized. Some emmissions added on some textures, bump + glossiness tweaks and 3 wellset lights...throw in a good choice of environment and its good to go.
This is the original upload to Poly: https://poly.google.com/view/3B8kUeUv8Kq
What you see here is the original in app upload to Sketchfab
https://skfb.ly/6XrFE
and this is after the optimization with the 3d editor
https://skfb.ly/6XrIo
Conclusion...with a little work it can really look good...
As for integration back into Tilt Brush, the Tilt file is in the download to be shared with other users...
https://help.sketchfab.com/hc/en-us/articles/202600873-Materials-and-Textures