First of all I would have really liked to see Guardiola managing an average team as well to see his quality better. Because he started his career with Barcelona B team. And the teams they have managed after that are only Barcelona, Bayern Munich and Manchester City. He has been really successful with these teams maybe. But I also agree that even an average manager would become successful with this kind of teams. Maybe he wouldn't be as much as Guardiola but the player quality would still help a lot.
I agree with your statement but not all, we have seen several times where average coach’s and also more successful ones coaching this big teams and still fail woefully. It cannot always be with the help of the level of players in that team, sometimes his tactical approach also matters in making those teams successful. You strategy and tactics will help a lot to coach any team, whether small or big team in football.
However I must admit a fact that he literally built Manchester City according to his will after coming here. I think this is a very special time period of his career here because of that.
Manchester City are practically not an underdog team, but his presence there made the team to become stronger and where able to go far and achieve all that they got today. First, he is an experienced manager and coming to a team that’s already made will only boost his confidence and make only little repair and changes in the squad. All he had achieved there are all well deserved looking at how far he has come.