Obviously, no one knows for sure.
As time goes by, each player will find his identity at the club. There is even a possibility that he could become captain. However if I could choose, of course I would prefer Valverde or Bellingham to be captain because these two players are clearly much more mature and have leadership qualities on the field. Mbappe is better off focusing as a goal hungry striker.
Bellingham may be a good player and he is quite reliable but I don't think he would be suitable as captain but we don't know for the future. Maybe things will start to look up when all these players have played more time for Real Madrid. Later you will definitely see who is suitable for it. And I think Real Madrid always needs a captain who is relatively older than the other players. And of course with good quality of course. And he must be a figure who is respected by all the players in the squad.
I don't know what you see in Bellingham to say that he's not fit for the captain, I will not argue with you because everyone have perspective of viewing things but for me I will not oppose him if he's been given. I like the way he plays and coordinates other team members, I noticed that when Vinicius was injured and the way Mbappe was playing, Bellingham thought he could partner with Mbappe to cover up for Vinicius' absence but it didn't work out, so he decided to put it in his shoulders to secure victory. Bellingham is cool guy and he easily adapts to the environment he finds himself, he takes his time to study Mbappe and his mode of playing and eventually he got. I consider him as a leader because he is one.