Hmm, I have no idea if coaches would use percentage to assign blame, that doesn't sound right to me. I would think a coach would tell the receiver you have to make that catch, as well as tell the QB you need to place the ball more accurately to increase chance of success. IMO, the majority of the blame on that play goes to the WR. However, the overall chance of success of the play increases if the ball is better placed.
If QB-X places the ball in stride 8 times out of 10 on that play vs. QB-Y placing it in stride 6 out of 10 times, over the course of a game/season/career, QB-X has the better odds of being more successful in the NFL.