Fairly strange, that. I tried several combinations, mounting with/without the ubi device attached etc, and only time that error happens is when you try to do it as user. As root, it never gives that error.
~# whoami root ~# mount -t ubifs ubi0:var /mnt mount: permission denied (are you root?)
~ $ whoami user ~ $ mount -t ubifs ubi0:var /mnt mount: permission denied (are you root?)