Fragments are the no 1 threat. The explosive power is pretty minimal, especially when using 60mm/81mm/82mm mortars. Some crappy chinese stuff pulverizes over three quarters of the steel material and creates only few big fragments thus rendering the rounds pretty much ineffective, but if you compare the effect to the effect of modern rounds like those used by US military or Finland, its totally another dimension.

Yes, a mortar could easily kill 27 people. Thye would all need to be within 15 meters of the explosion. I was in battle and we had one occasion where 61 enemy soldiers jumped into an excavated section in the ground to attack from. The one 60mm mortar we fired landed in there and killed all 61.