Well, I don't think anyone ever thought once that Mbappe was a bad or useless purchase, but it's simply that he had to find his place within the team, he's certainly young and is fitting in, in fact he seems to have really integrated,Now he just has to prove how many goals he can score
Well, for me I think his already fit into the team within a short period of time, yes he had a shaky start in the Spanish league but I think it was less than 3 - 4 months it took him to adapt to the team and the league in general and how his becoming a monster in front of goal for the team. Some people thought it would take him a season or two to fully adapt but it took him few months and his taken the league by storm as expected because he was already a big player.