Whilst it's wrong (he did win much), I also think it's right in a way, because he wasn't a good enough manager for that level. I think his tactics, buys, subs and treatment of players were all very questionable, and the only reason why he did well was due to his ridiculous budget.
On the contrary, Mancini isn't neccesarily responsible for player recruitment, that's more a responsibility of the board. Mancini outlined the players he wanted at the end of last season - Danielle De Rossi, Eden Hazard, Javi Martinez, Daniel Agger and a certain Robin Van Persie who fired Man United towards the title - in a late panic, Mancini ended up with Scott Sinclair, Javi Garcia, Maicon, Nastasic and Jack Rodwell - understandably, he wasn't given the backing he expected in the summer.
That being said Man City have been disappointing but it's worth remembering where Mancini brought them from and the progress they've made under his stewardship. Whilst they haven't been successful this season, it is worth remembering that next season would have seen him competing against David Moyes at United, regardless of the Chelsea situation. United will be undoubtedly be weakened by the loss of Sir Alex and therefore I do find his sacking incredibly premature. However, that doesn't mask what has been a disappointing season but ultimately, I feel he's a victim of his own success.