Exactly this. A guy who plays for one of the best national teams in the world with great players around him, and also plays for the best team in a much easier league.
VS
A guy who plays for a terrible national team, that he often has to carry.. and plays for a 'top 6' team in a much tougher league.. which he also has to carry at times (when Kane is out for example).
You can't easily compare them.. but if you do. You have to imagine them playing in the same set of conditions.. and in the same environment. And that is tough to do.. but it's a lot closer than you think.
Also.. Mbappe just like almost any other player has patches where he is non existent. E.g 0 goals and 1 assist in the euros, no goals in the Champions League for over a year at one point etc..