Yeah Grossman shouldn't take all the blame for that game. The weather obviously was a factor and definitely limits what you can do on offense. I'd think the rain would make it easier for a defense but the Colts offense just flat out dismantled the Bears D.
I also think the Colts D deserves a lot of credit for their coverage. It's not that Grossman was making bad decisions but he has no options at all on a lot of those plays. None of the receivers would be open, the running game(other than a few big plays) didn't really go anywhere, and the offensive line spent more time running backwards than they did blocking.