The Ostheer strength was support weapon, a nerf all mortar affect ostheer the most, the same way that if all HMFG was nerefed ostheer would be affected the most or if all light vehicles where nerfed ostheer would be affected the least. With 15 more range the ostheer mortar had a better chance to win a dual.It is a simple as that.
The ostheer mortar might be the "best" but the difference is currently minimal.
Mortars are for countering support weapons, and Ostheer has the best anyway, and depends on them the most as you say. That is the whole point. Ostheer is generally going to need a support weapon counter like the mortar less in general compared to other factions. Theirs happens to be the best anyway and whatever - thats cool. Nobody is asking for a nerf.
But good news is devs did buff counter barrage for the Grw34, so technically speaking it does get the extra range back in a barrage duel / counterfire role.
If you want more done with it specifically, however, thats a subject for the Ostheer thread.