Cannot access user/library/Mobile Documents files

If you visit Finder Settings : Sidebar and enable iCloud Drive, you will have a one-click view of your mobile documents in the Finder without having to navigate to ~/Library/Mobile Documents.

~ tilde indicates  your user library folder

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Or…

 Use the Command-Shift-. toggle in Finder to show or to hide hidden files and folders

Method 2:

1. Go to the Finder (or desktop).
2. Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
3. With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.

 4 Select Library from the Go menu (while holding down Option) to access the hidden folder.

1. Method 3:
2. Click anywhere on the desktop so Finder is the name of the foreground app shown at the upper left.
3. Press Command+Shift+H
4. 3  With your home folder open, press Command+J. This the only place you'll see a check box to Show Library Folder. Turn it on.
5. Method 4: Press ⇧⌘. —the command-shift-period chord—to toggle the display of the hidden files, and the Library folder will then become visible in your home directory. And will become invisible, when you press that chord again.
6. From now on, your user account Library folder will be visible without having to use special keys or commands to get there.

Oh, you're correct, for iCloud Drive...

Excessive size of Mobile Documents folder - Apple Community

Do you have iCloud Drive enabled?

What rights does it show?

I don't have any Mobile Devices, but can see into my empty folder...

Sounds like something to try.