Is there an actively maintained mobile UI for Django admin?

Since version 1.9 Django uses a former third-party app called django-flat-theme. There's another app made by the same developer which adds responsiveness to the admin. This app is called django-flat-responsive.

That means, if you're on Django 1.9+ just add django-flat-responsive. If you're running an older version of Django you need django-flat-theme and django-flat-responsive.

As user mirek points out, all of this is already integrated into Django 2+.

This is what it looks like:

Try this: https://github.com/douglasmiranda/django-admin-bootstrap

It uses Bootstrap and is BSD licensed