To the OP's points...
Along with big investment comes big expectation. City fans rightly expect our club to do very, very well. We have one of the strongest sides in world football, and IMO easily the best side in the P/L - so we should expect to win the P/L and advance far into the C/L. Too, we should expect some silverware. If we fail to do any of this, criticism is almost certainly warranted, barring multiple injuries/massive bad luck.
But this is the same at other clubs - even more so. In Italy, for example, players having bad performances - or so I've heard - receive death threats and cannot go out in public for fear of attack.
In Spain, expectations for Barca/Real are always sky high - failure to win the league and at least get 2nd in the C/L - is met with derision.
It's fair to say, that on the continent, fans are much more demanding - failure is met with far more dissatisfaction.
So no... I don't think at all that City have a negative fan base - rather we have an excellent squad, and our fan base expects corresponding success.
As for lack of criticism of Mou at United - just wait for Mou's melt-down 3rd season - the season where he completely loses the dressing room and attacks fans/players/management. Anti-Mou sentiment will be plentiful.