Yes, even when hiding, the casualties are way too high.
I know it's also a matter of opinion, but if two mortar tubes shell a squads's slit trench for a few minutes, I wouldn't expect them to cause more than perhaps 1 casualty, if at all - unless of course a very lucky direct hit occurs (In CM, the trenches are much too broad, leading to easy direct hits). Would this make mortars and other indirect weapons useless? No, you'd just have to just work with the suppression that your mortars provide! I dare say that this is also the more interesting solution from a gameplay point of view. Also, even this single casualty would lead to combat stress and recude the defenders' resolve/will to expose themselves to the incoming fire. But I stop here, as this whole issue is related to a wider debate about whether suppression and morale rather than the actual killing (which for various reasons seems to happen too easily/fast in Combat Mission) should play a bigger role in fire fights.
More generally speaking, quickbattles are in an odd place right now. The defender's main advantage are ambush situations, which happen all the time because the lines of sight/fire on all the quickbattle maps (not the scenario maps!) are way too short. So the first shot (which the defender usually gets) is fired at short range and is therefore decisive. Most heavier weapon systems are not even needed at these ranges. By contrast, the defender does NOT gain any benefit from prepared positions, as these can simply be wiped out too easily. In my opinion engagements would be more realistic and interesting if the balance was shifted: the defender should gain an advantage from prepared positions, while the attacker should have a the opportunity to bring his overwhelming forces to bear without them being wiped out piecemeal in super-short-ranged ambush situations. In such a setting, i.e. with longer engagement distances and more protective field fortifications, spotting the opponent first would not be as important and decisive as it is right now. This would save us all a lot of spotting fiddlyness and scouting paranoia. I'm tired of playing nothing but ambushes. Longer engagement distances would also affect the usage of artillery (if the opponent needs to cross more ground, you have enough time to call in arty before the enemy is back into cover; at the same time, you'd need to set larger target areas in order not to run the risk of completely missing the advancing infantry...). In a nutshell, battles would be much more "attritional" than they're now. Currently, quickbattles are characterized by fast rushes, rolling the dice, horrible squad-wipe-outs.
I don't want to talk ill of Combat Mission and its developers, as it is a marvellous game series that has brought me (and is still bringing me) a lot of fun and there is no game series even coming anywhere close to it. Kudos where kudos is due! But I really do wonder why that one very essential aspect of WWII warfare remains so neglected and under-developed in a game otherwise quite obsessed with detail and realism.
As it is, the best means against enemy artillery is a small map (so that no FO can get eyes on the target area without being instantly spotted and taken under fire). This does not work against TRPs though.