One way or another, everything's going well for City. Many teams are struggling, including Real Madrid. Therefore I think the City looks great. I think City fans should enjoy today, contrary to what you say. The next stage will be challenging opponents and tighter challenges.
Facing easy teams in the first stage is not something good generally.. In the next round, many games are already fixed and coaches can test their substitutes...
Would you prefer being in easy Champions League group or having difficult opponents? I don't understand how facing easy teams is NOT good. You can rest your key players and secure promotion to knockout phase soon. Manchester City had luck with a draw this year.
In contrast, Barca has difficult group and both Borussia Dortmund and Inter are serious contenders for 1st place in the group. So Barca will need to play with their best team, risking injuries and so on.
The only reason why being in difficult group could be beneficial is money from ticket sales. Man City didn't sell out their matches against average teams like Dinamo Zagreb or Atalanta, while Barcelona sold 86000+ tickets against Inter and it will be similar against Dortmund.