Even the worst compared to the entire Real Madrid starting squad.
Mbappe has just moved to Real Madrid and he isn't used to play in top league like La Liga comparing to the lower level of Ligue 1. He will take some time to adapt and will resume scoring.
Besides, I think his score is lower because people expect a lot from him, and to score a goal at least which doesn't happen so they are a bit disappointed and this explains why also Valverde has a good rating because he scored a goal.
Basically Mbappe is used to tough competition, and playing at PSG or in Ligue 1 is also not a lower level than La Liga. Also, basically Mbappe is used to playing with great demands when PSG plays in the Champion League and also when Mbappe plays in the French national team. So, with Mbappe getting used to playing in the highest competition then of course, it is natural for Mbappe should to play well quickly, because Mbappe is also always included in the starting eleven.
So, it seems you are wrong if you think Ligue 1 is a league below level then La Liga, because Ligue 1 is also included in the top European leagues. With the low rating that Mbappe has so far at Real Madrid, it is certainly a bit of a disappointment too, because Mbappe is basically a top player anyway, but Mbappe still needs time to adapt. Because as far as I know, at least the top players who have just joined or just played at Real Madrid, at least they managed to have a good contribution in a short time. But, when top players need time to adapt, then maybe players like Hazard in the past also became one of the transfers that in fact failed to provide good results for Real Madrid.