I meant 2012/2013, and I'll talk about this season later on. Even in his best season, Hummels still made positional mistakes - that is to be expected because he was still maturing. Away from that, when it has come to the massive games in European fixtures, he's made some catastrophic mistakes. He had about two good games in Euro 2012 and then turned to mediocre vs. Italy. He's a player than can be naive and has shown his naive side frequently since Borussia Dortmund last won the league.
I see "tactical fouling" as an irrelevant factor in this debate. This is a poor point IMO because Hummels' mistakes often lead him too far astray to actually make any tactical fouls, so that point doesn't make sense when you look at it this way. See this, for example:
In Europe, Hummels has been an utter disaster. Just before I go on, I wasn't really criticising him for this season because it's early on, but if you want me to dig out an example - I'll also post examples last season - then look at the first video below. Hummels leaves his man - for the first goal. At one point, he was next to him, but then he loses focus and Schmelzer has to go out and attack the ball. In the second video, Hummels isn't quick enough to block the cross and Benzema scores - if you want, you can say he's partially responsible. For the second goal, he doesn't connect well to clear away the cross... and again, Real Madrid scores. In the same match, he made a couple of errors in the first half, one of which led to Ozil with a one-on-one. And in the Champions League final, he was a disaster. Most of Bayern Munich's attacks came down his side. There are more positional calamities I can post, but I don't want to spend my time cutting clips just to prove a point.
In your third last paragraph, you say that he couldn't have become such a talked about centre-back if he had bad positioning. Just to clarify, I did not say Hummels is mediocre, but what I did allude to is his poor positioning these days. You can become a highly rated centre-back, but still have positional errors. I also don't like how you're telling me to show you the positional errors he's made this season, which I didn't really argue. If you want it that way, I can give you torrents and if you don't let your bias dictate the outcome, then you will see that he's also made some poor errors this season.
Hummels is a very good centre-back, but at the moment, he is nowhere near the best or Europe's elite list. His fall has been abrupt and he looks off the pace, but that does not mean he's average, or whatever. It means that he's not the finished product.