Opening Mobile Documents folder in Finder on Sierra
I found a really nice type of workaround for this from here. Essentially you need to run the following command in Terminal:

ln -s ~/Library/Mobile\ Documents/ ~/Desktop/Real\ iCloud\ Drive


This will create an alias on your desktop that shows all the folders in the Mobile Documents folder when you open it. You are free to browse it to your liking—albeit with one caveat: you can't directly open any folders. You'll have to use the list view's ability to expand folders and then view files from there.