I disagree.
If its a braindead unit you would see it every game, wich doesnt happen. I am not sure on the numbers but its damage is not amazing among mortars, its sounds on impact however, making it seem awsome. If it does one shot at all it massive rng roll. And that for a pop and ecpensive mortar.
There are plenty units abilities that can one shot far more easely then the 120mm can.
It is used alot of time and in certain maps more than others, and it depends on your lvl to see it or not see it much offen, that doesn't mean it doesn't need a nerf, and its accuracy is actually fine especially with barrage, in addition to that its AOE is pretty big.
u should check Vipper reply if u want real numbers.