That would depend where a person believes the lionshare of the blame is for the passing game troubles.
There's enough blame for everyone. Fisher, the OC, the O-line, the receivers, injuries, and of course the QB who always gets too much credit when things go well and too much blame when things go poorly. The NFL is the ultimate team sport and requires cohesion by all involved. If one element is out of whack, all will suffer.
But Jeff Fisher gets the ultimate blame because the buck stops with him. He hired the OC who is responsible for the game plan each week. He, along with Snead, drafted or signed the current players we have. He hired all the coaches who are supposed to train those players. He is responsible for in-game adjustments, etc.
Foles, Keenum, Bradford, Hill, and Davis all struggled under this system set up by Fisher, and all QB's who the Rams use after this will have the same problems; a weak, injury prone O-line, receivers that either can't get open or drop the ball too often, a simplistic game plan that fools no one, boneheaded penalties that stop drives in their tracks, etc.
The common thread here is Jeff Fisher. Season after season the offense plays poorly. A normal owner would see this pattern and take the appropriate steps to fix the problem. But here we are...waiting.