Partly surely but not all the blame can be put on him. No one could have foresaw Garrosh would go on a path of genocide. Nor would he have foresaw that the level headed orcs would be so spineless in their objection to such ideas (I'm looking at you Eitrigg) nor would he have predicted how bad the orcs were craving bloodlust and would swarm to Garrosh's ideas. All in all Garrosh is mainly to blame because he is the one fucking things up, but Thrall has to take some responsibility for what happened. Not fulfilling his promise to his people to return once the Cataclysm was over is a big deal as well.