I disagree with both of you.
Rafa did not play bad, but his average level is not great on grass, definitely not top 10 material, but would not classify it as garbage either.
Muller was not playing above his head, it just happened that last year he finally praked, won finally titles on hard, and played with a lot of confidence. In that form he was better than average Nadal.
Among the losses Rafa suffered at Wimbledon, this was a better showing, because he lost to a quality player on grass, much more acceptable than the losses to Rosol, Brown or Darcis.