The mortar knockback is always the same, it must just be the way you are DIing it.
When hit by the mortar, it will send you in the direction that snake was facing when he fired the mortar round, but at a high arc trajectory (straight up for the most part, but to the left or right a decent amount) , no matter how the mortar was falling, raising, or on which side. So if snake was facing to the right when the mortar went off, you'll take knockback going to the right side of the stage.
Plus, you can really only DI the trajectory in a couple fashions. Say you are facing the mortar and it gives you the knockback towards the staright up, but to the right, then DI right and you'll go more horizontal, DI left, down or Up and you'll pretty much go as vertical as possbile.