Bringing in Messi could be a very strategic step for Manchester City in strengthening their attacking line, because messi have extraordinary experience and abilities, which can have a positive impact on City midfield, his presence can certainly also increase team morale which is in bad shape at the moment, however, there are several things that need to be considered by manchester city before recruit him, such as age, his adaptation in the premier league which is rely more physical than MLS, but apart from that, if Messi can adapt quickly, of course his presence in guardiola squad will be an effective short-term solution to overcome the problems they are currently facing.
I think it won't be a good idea to bring Messi to Man City. He is already 37 years old, his performance is no longer as well as in the past. And his fitness is already declining a lot. Messi isn't suitable for EPL competition, EPL needs a player with strong physical and good fitness. However, I agree that Messi may improve the mentality of the players. But he is unlikely to improve the strength of Man City midfield or attacking line.
Why Man City doesn't try to sign a player like Florian Wirtz or Jamal Musiala? These players are in their best ages, they are also in their best performance. Since they are still young, they also can be the assets of Man City because they will have a more expensive price in the future.