Well in my opinion I think the problem with Kylian Mbappe is about adaptation, Kylian Mbappe was a very important player for PSG and also an important player for the French national team as well, though he hasn't really impressed the Madrid fans ever since he arrived at the Bernabeau but with time I think he would play like the Mbappe we all know. Sometimes players find it difficult to settle in their new teams and Mbappe wouldn't be an exception but I believe with time the player would start performing well.
Mbappe is a good player and has potential, so what is happening with him right now besides the adaptation problem, we know that many Real Madrid players are injured and the midfield and front lines do not have good chemistry. This is why Real Madrid is currently inconsistent and this also has an impact on Mbappe who has not found his best performance. It's just a matter of time and I think Mbappe will find his performance again soon.
Moreover, Real Madrid against Atalanta is a very important match for Real Madrid to qualify for the last 16, they only have three matches left and the match that is quite strong is the match against Atalanta. If Real Madrid manages to beat Atalanta their chances of qualifying are very wide open.
My prediction is that this match will be won by Real Madrid even though Atalanta is on fire. Because I believe in Real Madrid's mentality, in the early season match Real Madrid beat Atalanta with a score of 2-0 in the UEFA Super Cup competition. At that time Atalanta lost, Real Madrid's winning goals were scored by Kylian Mbappe and Federico Valverde. So I think they will do it again, especially Mbappe who needs confidence.