I've given this some thought and lol we have another mortar man on the thread other than me so you don't have to take my word for it. Believe me, I'm thinking about it. The IRA (in Ireland) have perfected the one use, throwaway mortar. Its ballistics are worked out beforehand and its accuracy is left to that chance.
Practically speaking, I don't see how a mortar could be employed to reasonable effect without some training involved. Would an FO be needed? Yes and no. If the mortar man were knowledgeable in his craft he could expect a reasonable amount of accuracy regardless of whether he had an FO or not. Another thing that he could do is to adjust his fire on a target totally away from his "real" target until his guns were "registered". As long as this registration was accomplished within certain parameters his gun would be accurate when traversed onto the other target. I'm not saying this is the case.
But if I were a mortar man playing the Che Guevara role in Iraq, I'm sure I could land a few rounds on my target.