One of the reasons why Mbappe is underrated in this award by many people is team achievements, his team didn’t win any trophy last season despite scoring highest goals in almost ok the competitions he played; and his national team France wasn’t able to also reach the third position as they lost to England, yet, he scored more goals than Yamal in all these competitions.
Everyone have his favorite players among these players, and all of them will bring reasons for these players to be worthy of the award because all the listed top four players are good and had impressive season across all competitions. For now, we can’t say, but I think they luckiest player will win it.
Exactly, what's affected Mbappe is club achievements. Sometimes i reason Mbappe issue having hard luck of winning champions league when playing for PSG. Since he arrived in real Madrid they where unable to win laliga for 2 seasons now even with the goals he scored putting all his efforts if they can win laliga.
Every individual have their favorite players, just as you said those 4 player's did very well. But for me since Harry kane and yamal are in first and second they should give one among them especially kane, because yamal still have all it takes to win it in times to come.
In terms of personal achievement Kylian Mbappe is qualified because of his goals prowess but when it comes to club achievements, he hasn't been able to achieve any title with Real Madrid and that is just what has affected his chances of emerging the ballon d'or favourite. Mbappe was just unfortunate at Real Madrid, if he had continued with PSG he would have won the Ballon d'or and still have 2 Champions League title to his name.
Lamine Yamal and Harry Kane are the key target for the most decorated award but Lamine Yamal stands a better chance because of individual performance and club achievement, he even achieved the World cup title with his country Spain, that's enough to win him the award.