This isn't about strategy. I know (at least in theory) how to play the game though my micro is only now getting better. The questino is balance.
Yes Supply drop give you munitions. But you are trading in 100 mp for it, the most dear commodity in the game. Mortars are not "free", they cost 81 mp (yes, a bargain) but they are fragile. They are also limited in speed an use, and by late game of more limited utility. But no, they are NOT artillery.
Mortars are only "guaranteed" effective (as in they do what you want/need) when laying smoke. They might be able to crit a vehicle if you get really really lucky. It cannot do what all the other artillery do which is area denial or fortification/base bombard. Not to mention that anything US gets from the 61mm mortar the Axis gets even better from the 80mm.
Instead of going in circles, lets stipulate that either the mortar is a piece of artillery or it isn't. If it is, let's just take the Stuka and the Nebel from Wehr, since they already have artillery and it is better than the US artillery.
It should be. Mortar has always being seen as one kind of artillery, isn't it?
Area denial and fortification/base bombard? Surely it can do that though not as effective as other arty. Still can help you bombard units, bunkers, 88 mm, flak 20 mm, and/or draw enemy arty from your other units.