It doesnt matter if you can see the target or not. The cover rules are clear, and not being able to see the target doesnt grant them cover. Unless a special rule says otherwise. Mortars dont have any special rule saying otherwise.
Q: When determining whether a model benefits from cover, does
the model’s entire unit need to be fully on or within terrain, or
just the model making a particular saving throw?
A: All of the models in a unit need to be at least partially
on or within terrain if any of the models are to receive
the +1 bonus to their saving throw.
In addition vehicles need to be at least 50% obscured from the firers view. Which, of course, is true, if you dont have
LOS.