Here's the problem with MW strategies against Mortarion.
Let's say Mortarion starts in his deployment zone, far to the Left. His opponent has to shift many units left to get him in Smite range, leaving the right side less protected. DG units take objectives.
Let's say he starts far to the left, but his opponent doesn't commit a lot of units to crushing him early. Mortarion takes some MWs while DG units engage with the rest of the army. Mortarion remains in the game until at least turn 3, taking several large units with him.
There's a case to be made the least optimal strategy for using Mortarion is charging up the center. He can draw psychic-heavy armies off objectives through presence alone and that leaves a lot of room for DG infantry to just sit on objectives.