I do think that he's better on grass and his peak is definitely higher compared to his highest level on clay, the combination of serve & return he possesses is lethal on the surface, especially if a player is a phenomenal athlete (as Bjorn Borg had demonstrated).
Let's not forget that he lost at RG during his best years (2011, 2015) to the players not named Rafael Nadal, so he's more vulnerable on a red dirt, imo.
I mean it's his second most successful, yes. He's still a far more comfortable and natural clay court player. If he had to play on grass in a way that was significantly different to the way he plays on clay then he probably wouldn't have 4 Wimbledon titles. Like Nadal and his 5 Wimbledon finals he's capitalized on a period of time in which grass courts have less top tier competition than all the other slams. There's a reason that every other major save Wimbledon has had a winner outside the big 4 going back to what, 2003?
His game at a first glance may not look tailored for grass, but it actually is very suited for it.
And like Kishnabe said, he counters grass specialists.
-GOAT returner. I've never seen anyone able to read Roger's serve as well as he did in the 2014 and 2015 final.
-Penetrating groundstrokes. He doesn't hit the hardest, but his ability to get the ball deep in the opponents court makes their life hell. It's hard to hit balls when they're skidding near the baseline ON GRASS.
-Ability to slide on grass, very few players in history have been able to do this consistently. This gives him an immediate edge, even if he isn't as balanced as Federer or move as well as him
-One of the best passers of all time
-He actually has a extremely decent serve for someone his height. He hit 23 aces against Nadal, who is a great returner. His 2nd serve also has a lot of kick and pace. In terms of being a placement server, Novak is up there.