Well, sure - Mort can do stuff that Harry can't, because Harry's talents don't run in that direction. (In game terms, Harry doesn't have any Ectomancy foci or specializations, while Mort would necessarily have Ectomancy foci since he doesn't have anything else to use his focus slots on. Also, Harry's highest spellcasting stat is Conviction, which isn't very useful for Thaumaturgy.)
That doesn't mean Mort would be better at Ectomancy than a wizard who was actually good at Ectomancy.
Now there is a reference in GS that implies that he's Council-level, but I don't know what that means exactly... does it mean that he could train into a broader ability, or he has enough "raw power" for the Council but couldn't pass their tests since his talents are narrower than what they're looking for, or what?