Yeah the mortar is really inaccurate when upgraded. However, it gains massive range and the arty size shells to make up for the greater scatter. It becomes much less useful against moving blobs but much better against bunkers, paks, MG teams and other defensive positions. Unless you need it to take out an enemy mortar because it probably won't hit.
Is there any difference in the trajectory of an artillery round and a mortar round in this game? That might be the cause, because of how the game sees it as an artillery shell and not a mortar shell so there is a different flight of the round.
Yeah the mortar is really inaccurate when upgraded. However, it gains massive range and the arty size shells to make up for the greater scatter.
That's not exactly empirical evidence.