It sounds like the Mortar is doing it's job, harrashing and forcing people to reposition, open points where USF can attack the enemy.
What's wrong with that?
The fact that one specific faction's entire infantry design relies on not being mobile. Before the mortar, this was countered by having the usage of grenades cost ammunition, thus effectively limiting the number of grenades and weapon upgrades that could be used by the USF player.
The mortar doesn't have that constriction, its only downside being a comparatively small amount of field presence being lost, which has never been a problem for USF in the first place and isn't one now.
In turn, this results in ever more double-BAR Riflemen down the line, which more than just laugh at LMG Grens, especially as the BAR works like an Assault Rifle in the game (with it being able to fire on the move) in all aspects. That would be okay, if there was some sort of trade-off available to deal with that newly found power.
And here comes the crux: Ostheer doesn't have that capability. There is no unit that allows you to take over initiative (in the "Kriegsspiel"-meaning) from your enemy. Or better worded, there are such units, but getting them is basically impossible at the time you need them.
I once discussed this in an earlier post, so I will go straight to the result: It is pretty easy to force Ostheer down a particular path, to dictate all their possible moves by assuming control during the early stages of a match. Sure, a good player won't let himself be forced to make a specific move at a specific time, so the spacial rules differ each match. But what kind of move is taken at what time is not up to the Ostheer player anymore, and that's why Ostheer feels so constrained.
The problem is, that the mortar is only a symptom here. This is nothing new, and it is nothing we haven't seen before (unless we didn't want to see it). This problem has existed from the beginning, and against the Soviets it was balanced through sheer DPS calculations. That worked rather worse-than-good, but it worked. In come the Americans, who further solidified on the Soviet idea in terms of execution (which is why USF feels more fun to play as, compared to Soviets, despite the Soviets being the better fleshed out faction), and the Tommies, which are Ostheer all over again, except without most of their weaknesses - most importantly the lack of a German faction that could capitalise on British weaknesses, a German USF faction so to speak.
Fixing the mortar, properly fixing it I mean, not Relic's idea of adjusting a few values, won't do anything against the basic problem. What happens next time USF gains a unit (through a new commander, just as a gedankenexperiment) that offers similar capabilities?