Sadly, sometimes we need to pass some time to configure the machine as we want.
I guess, mate, disallow or didn't implemented te blank ssh file integration for security reasons.
As @dandnsmith said, the best for you will be to install UM on a SD card, next boot it and configure with plugged in keyboard & mouse ( i guess openssh-server is missing and need to be installed ), next use it.
If you installed ubuntu mate on this rpi, means you will probably use it's GUI ( otherwise, a ubuntu server will be enough ), that also means, that there will be a keyboard and mouse plugged to it later.