At the moment there is no place in the web user interface that repository owner can tag a commit. It is a waste of time and data traffic to clone the whole thing on your local drive, tag it and push it back !
I have a project that has a public repository and a private one. In private I work on my new ideas and on public one I encourage people to fork it and develop it. I don't have any local copy of the public repository and I create pull request from my devel repo to the public one. I can tag the commits in my devel repo but after the pull request is applied the tags are not going to show up in the public repo, so I think having this option would be good.