It is not that Manchester city is bad but what happened is that they dropped in their performance which was at the climax. Even as they dropped in their performance, they also win matches, so the slight difference between their performance this year and that of last year is what is considered by many people as bad. Even if Manchester city is no longer playing in their peak, they have been able to win their champions league group matches and so, they are still good
People always want to see many goals scored by the aggressive team, but they forget if leading two or three goals are already enough since it's already hard for the opponent to comeback. If Champions League is the only league that Manchester City play, I'm sure they will perform all out.
But they're lack of players and they have a busy schedule, so it's make sense they're not want to try harder when it's already a sure win, they can save their remaining energy for the upcoming match.