I think they did a little, to be fair. Chelsea were never fully convincing this season, the odd good game here and there, but not enough. City were absolutely brilliant at times, but they just seem like a team who were fine as long as there was nothing on the line, especially in the first half of the season. They haven't been anywhere near as good at crunch time. Lost the league, lost to Wigan, and didn't perform well against a less than great Barcelona team.
I honestly think that Aguero's injury problems are being understated here. If he'd stayed fit they would have run away with the league imo, they were on fire until he got injured against Spurs. In all competitions in the 5 games leading up to, and including, that Spurs game Aguero scored 7 goals, whereas in the 5 games after the injury City scored 3 goals. Aguero hasn't scored since. Interestingly, Negredo hasn't scored a single goal since that Spurs game either, I don't think that's a coincidence.
The standard of opposition in the 5 games before and after the injury respectitvely definitely play a part in those figures, but 1 goal in 180 minutes against Norwich and Stoke would suggest that they aren't entirely misleading. Aguero is as important to City as Suarez is to Liverpool imo. He missed the Chelsea, Liverpool, United, and Arsenal games in the second half of the season, imagine Suarez missing all those for Liverpool. It doesn't matter how much money you spend, you're always going to miss your best player.
In terms of Chelsea's challenge, every time I've watched them since January I've thought they were missing a player like Mata. Willian, Schurrle, and Sala are more suited to counter-attacking football imo. When they have to break teams like Sunderland down their pace is of little use. Their strikers get a lot of stick, but in fairness to them they don't always get the best service, especially when Hazard is missing. I just can't get my head round letting their best player go midway through the season, I personally think it's cost them the title in the end.