This.
I don't even play Blue Mage, but I know there's not a single monster in the game you can land that doom on that will be worthwhile.
Any monster it can land on, you can kill it normally in less than 60 seconds, and without wasting as much MP/etc.
In short, I don't know why Mortal Ray was decided to be a learnable spell.
It would have to be objectively broken to be useful.
And, otherwise, it's a waste of a spell slot.