It is obvious that Roger has the disadvantage in this era of fewer grass tournaments to play, which is much complained about by his fans. And has to be accounted for. However, this doesn't make up for Federer having lost more on grass than Rafa has on clay. If anything, it proves the point. There are so many more opportunities for Rafa to lose on clay than for Roger to lose on grass. But there it is. Is it easier to lose on grass than on clay? I think we'd all say yes. One drop of serve and that could be the set on grass. But I don't think even that makes up the difference in their results. Remember that most are trying to make Roger the overall GOAT, as well. He would be held to a high standard on grass, and he's had more stumbles than Rafa on clay...by rather a lot.
As I've said, there are more opportunities on clay, so, yes, the grass one is impressive, but so is 81 on clay. That's a close one, if you put it that way.