I can not fill any task/bug form on any Phabricator instance on my mobile. I would expect this to be working.
Not, sure, but maybe https://phabricator.wikimedia.org/ does help?
Open Maxthon or Firefox browser, open any form (like Create new task), place the cursor into placeholder field and start type.
See also my original task to this issue here: https://phabricator.wikimedia.org/T209045