As of Sierra(?) opening
~/Library/Mobile Documents in the Finder actually takes you to
~/Library/Mobile Documents/com~apple~CloudDocs. This the Finder stylizes as "iCloud Drive" and only has folders for certain applications.
It is still possible to access Mobile Documents proper through Terminal, but even when opening the directory in Finder directly from Terminal it redirects to com~apple~CloudDocs.
Is it possible to access the root Mobile Documents folder from Finder?