Mourinho takes over clubs who are either rich or already have a great squad, so if he had lost 6-0 then the reaction would be more extreme, and rightfully so. Wenger's Arsenal may have lost by large margins to rivals this season, but they have been solid defensively in most of their other games, which is impressive considering the price of the squad.
I personally don't remember everyone expecting Arsenal to title challenge after the Ozil signing, although I may be wrong of course. However, Ozil is only one marquee signing, where the likes of City and Chelsea have a number of players of his stature, so it's unrealistic to expect Arsenal to challenge for the title just because they've signed him.
More of the 'Mourinho only can succeed with big money and great squads' rhetoric I see? *Yawn*