It begs the question why would you want it mobile?
Two reasons, a minimum range of 18 inches and the special rule of 'Fixed' which applies to medium mortars. Having the ability to reposition a medium mortar to the most advantageous location as the battle develops is very useful. While I will concede that mortar would have most often been deployed outside of the half-track, there is precedence for it being used within it as well.
With a full load of 66 of the 81mm mortar rounds, this platform has the ability to cause the enemy more than a little bit of grief over the course of a battle. It should go without saying (and yet I will) that a Forward Observer team would be a requirement to fully utilize this weapon.
In all the Germans built nearly 20,000 of the 251 series of half-tracks, seeing action in all theaters. The Ausf D was introduced in 1943 and featured a simplified front and rear armor shape. Weighing in at 3 tons, with 12mm of armor, maximum speed of 33 mph, 124 mile range, capable of a carrying a squad of 10 men it was one of the most advanced half-tracks at that time.