When outputting any image from Lightroom mobile directly to any service, the image needs to have, at minimum, a © NAME simple watermark placed upon it. I don't think we need full functionality of the Watermark editor in Lightroom desktop but we do need the minimum functionality of the Simple Watermark in Lightroom's Export dialog. Until there is a way to mark an image, why would anyone consider using LR mobile to post anywhere?
NOTE: The original post was a request for a simple, text based watermark which has been implemented. This thread will continue to be the place to vote for image based watermarks.
Edit: 1/21/2020 - This thread has been changed to represent all three of the Lightroom Ecosystem Clients (Desktop, Web, and Mobile)
Edit: 5/15/2020 - This thread has been updated to include information on new functionalities. Please see the official post below.