Maxims are more vulnerable to mortar fire because of the way in which mortar mechanics work in this game. Basically what happens is the mortar shoots, at this point in time it places a marker on the squad that it is shooting at(or barrage circle). The shot then "scatters" away from that marker based on the distance traveled, vet, bulletins, etc. A deployed Maxim is stationary, meaning it will get hit a lot more often than say a Conscript squad that is walking around - and could easily walk outside the scatter AoE without even realizing that it is getting shot at, simply because Cons are almost always going to be on the move. You don't even have to use the barrage on the Maxim, you can just keep right clicking with your mortar selected, or attack ground where the maxim is meaning the maxim squad is going to have to relocate constantly. So yeah, long story short, all set-up teams are more vulnerable to indirect fire than normal infantry squads.
Just gonna add for anyone who might get confused: setup teams take increased damage from small-arms fire but not from area-of-effect weapons. That in no way nullifies your point that Mortars work quite well vs. Maxims, especially with cover being so much more important since the weapon profile changes I build Ostheer mortar almost every game.