a mortar wouldnt do anything against a mech or a tank. Unless it was one that the battle armor carried and set up (aka a BIG UN)
and real mortars dont act like the ones in mechassault, they are bombardment weapons for supressing conventional infantry. They are cheap and easily portable, and good for close support artillery in an infantry vs infantry situation. However MWLL is not based on battletroops its based on battletech, and has NO conventional infantry. A mortar shell would hit a mech or tank, make a little boom (compared the other Battletech weapons) and perhaps scare the pilot if no targets are in view. However I cannot forsee a situation where a manpack PPC or something would not be vastly superior.
Think of an RPG in modern combat. Shoot it at an Abrams or T-80 or something and it wont do crap. Now try arcing a mortar into a tank. Impossible unless your in the fighting seebees, and it wouldnt do much. I imagine armor technology is better in 3050 too. Automated turret ruining your day? Call for air support. Dont ditch your primary armament for a weapon thats only good at that.
Maybe a battle armor grenade launcher is what you all are picturing. In which case itd be good for messing with other battle armor, but once again itd be an inferior weapon to a standard laser or something (due largely to ammo constraints)
I dont see why programing a weapon like this would be worth it, unless the devs made it stupidly powerful for what it is.