Is there a Shortcut for "Mark all messages as read" in Outlook?

You could put the "Mark All as Read" command into the Quick Access Toolbar like so:

After that you can access the function via Alt+3. The number depends on the position where you place the command. Press Alt by itself to make Outlook display overlays for all hotkeys.

Keyboard shortcut

Click any message in the message list, press CTRL+A then press CTRL+Q.