If you watch Mbappe very well when he was playing for PSG you actually know that he is a good player I think he's just having a bad season in Madrid because of the little adjustment in the team and currently he's still trying to adapt to the playing pattern of the club
and I think he will get better very soon and the goals will start coming. The position of the player should be decided by the coach not us if the coach feels that Mbappe can't function well from the left wing then he should make the change and make his team to function very well. Right now I think it will be difficult for Mbappe to be seen as a great player when Vini Junior is still there I think the co-operation will not work until the manager steps in.
The problem I see with your opinion is that we are not discussing about a 17 year old player here who is a super talent and now has some problems adjusting to the new team. We are talking about Mbappe and while it is ok to have a bad game or two bad games in the beginning of a season, Mbappe is playing far below his best and is disappointing a lot of expectations so far. He is scoring his goals (mostly penalties I think), but he is not the one decisive player who made Real Madrid much stronger. It is quite the contrary. They don't look as if they improved a lot at all. But you could say the same about Vini who is playing a good game and then another bad game. They lack a stable performance level.