It is more a case of the problems with freezing and thawing of propellants. Best case scenario they work but not as well, worst case they either don't work or work "too well".
Unless one is careful when they freeze powder or powder-like propellants, it is easy to get condensation from the atmosphere this, when thawed can adversely affect propellants.
The problem is that mortars are designed in such a way that to operate them, the projectile and propellant bags are held above the opening of the tube, then dropped. Anyone who has fired a mortar knows that it does have to be lined up kinda carefully or one risks hanging the round - bad thing.
This lining up is pretty easy for a human, but rather hard for a mechanism. This is why automated mortars don't operate like human operated mortars. In order to use ice, some sort of holding frame would need to be constructed to hold the round, aligned with the tube, in such a way that it is released evenly.
Ice does not melt evenly. The only way I could see this working is if the round were held in some sort of clamp which is somehow released due to ice melting.
Way too complicated. They key to improvised munitions is keep it simple. Simple works, complicated don't.
If I wanted to remotely fire a mortar, I would construct a ramp like cradle and suspend the round from a string. Then choose one of the thousand ways to cause a string to break and Bobs your Uncle.