This may seem scary because it targets Mark's sanity, which is low. But I think that, on balance, this is one of the mildest weakness any investigator has. I would never trade this for Cover Up, The Dirge of Reason, or even Smite the Wicked were the option available.
Of note here are a few things:
1) Since it says "for every 2 damage," Shell Shock rounds down. 1 damage on Mark = 0 horror from Shell Shock. The great majority of the time, this card will deal 0 or 1 horror. Not scary at all.
2) Mark was already incentivized to keep the number of damage on him to a minimum because of how Sophie works. 6+ damage is the point at which Shell Shock starts to get awful, but Mark can avoid ever getting into that position. He should be aggressively killing monsters before they attack him, he should run healing cards, he should be playing assets to soak up damage for him, and he should be judicious in his usage of Sophie. If you're playing Mark sensibly, it's very rare that Shell Shock will deal more than 2 horror.
3) Shell Shock is extremely predictable (unlike, say, Final Rhapsody.) You know exactly how much damage it would do if you were to draw it. That means if you're ever in the death zone for Shell Shock you will know that ahead of time. You or a teammate can then heal some damage and/or horror (and most Mark decks are full of cards that do this), or Mark can play an asset that soaks horror.
4) Shell Shock doesn't deal direct horror (unlike Abandoned and Alone). All Mark decks are going to want to play horror-soaking assets, and quite possibly two or more of them. This means Shell Shock can still be manageable even if Mark has accumulated a lot of damage.
5) All in all, Shell Shock is one of the most likely of any player weakness to be discarded for no effect (other than wasting a draw, as all weaknesses do) or a very minor effect.