I agree, according to statistics (either the first 15 games or 20, I don't remember exactly) Flick's percentage of points scored is worse than Xavi's. Bright games mean nothing if the team plays poorly on average. At the moment (and it's far from the final stage of the season) Barcelona is out of the title race.
Assume it is correct and Flick performances in first 15 or 20 matches are worse than Xavi's record, it doesn mean he is a worse coach than Xavi. The same if Flick performances in first 15/20 matches are better than Xavi's record, it's not enough to say Flick is a better coach than Xavi. We must assess their performances at a season ending, and even in a longer time more than one season.
Both Flick and Xavi are good coaches but they can have up and down in career and I don't know who is a better coach, at least now.
Moreover, it cannot be said that Real has turned into a monster with the arrival of Mbappe - on the contrary, the team has many problems and is in the "setup" stage.
Changes in ranks of La Liga are from Barcelona themselves and competitors like Atletico Madrid and Real Madrid. Barcelona actually have many bad matches recent weeks and if they had played well as same as early of season, they would have not lost the first position to Real Madrid even Real Madrid had considerable improvement.