As Blaugrana said the big match sentiment isn't an issue, and there are enough counter examples of that, thee already were at Madrid (assists in Classicos, vs Bayern, goals vs Dortmund, Atletico etc). Examples from this season are also existing. But when the whole team is down, Ozil isn't carrying them on his own. That's quite common among many players, Ozil is just getting the biggest heat for it.
His performance ratio in big games isn't either bad or good, it's simply normal and dependend on wether Arsenal wins or losses. I remember he had Clasicos where he tore Barca to peaces, (Copa-final, Reals league winning classico and some others I cant remember) and also some where he was barely on the field.