Well, it's less lethal than high explosives or bullets.
Despite being used in a huge % of shells and in vast quantities in WWI, the percentage of deaths caused by chemicals in WWI was remarkably trivial compared to good ol' high explosive, and simply getting shot.
That depends. While a well aimed bullet or HE may inflict significant damage or death to a bad guy, mustard gas, or blister agent in the current parlance is potentially MUCH more lethal than either. The largest factor is the prevailing weather during the deployment of the gas. Deploying and agent such as this on a dry, calm day on a target that is suitably grouped (e.g. a command element, massed troops, or an encampment can be much more deadly than the equivalent number of HE rounds.
As a former 0341 (USMC Mortar MOS), a 120mm mortar is a large round. The contents of this type of round are dispersed in a fairly compact area, but mortars are typically employed as a group of tubes firing multiple rounds - one after another. Although more difficult to do with 120mm mortars due to their size and weight, a reasonably proficient mortarman could have four or five rounds in the air before the first hit.
So, if the Iraqis deployed a typical mortar platoon of 8 guns, they could have had approximately 40 rounds hit the target in a very short time. And that, my friend is a GREAT deal of blister agent.
As a Marine, I attended several NBC (Nuclear, Biological & Chemical) Defense and decontamination classes. As part of the training we got to see video of some Australian prisoners who took part in a test of blister agent and its effects on the human body (one would assume that they got some sort of commutation in return, but I am not certain of that fact). One test that was performed was to take a straight pin, dip it into the liquid blister agent and then apply it to the skin of the prisoner. The "contents" of that small pin was enough to cause the persons entire forearm to turn into a huge blister. Imagine inhaling this stuff - death is caused by the person actually drowning in their own fluids.
Look, anyway you slice it these are not "trivial" weapons by any stretch of the imagination. Deployed in ideal circumstances, round for round, they can seriously impact much larger groups of people than either HE or smal arms fire.