Yes he is a bit underrated among all the star players, I also don't no why is like that but it could be that the club he is playing for is the reason why he is not that noticeable because if Chelsea were to be very competitive on this premier League they would have seen more of Palmer but how can a player be ranked higher when they are not consistent, so inability for a club to be productive can have a negative effects on a player, so like I already mentioned if Palmer find himself on Manchester City his performance will even be better compare to this because the young star is very good and I think Chelsea will always look forward in trying to renew his contract.
Sometimes the right partner is needed to achieve top performance. We still remember how phenomenal Messi was when he played with Barcelona but not when he played with Argentina well, that's an example of how someone can't play optimally if they're not paired with the right players. Palmer may be in this situation so he can't be consistent, as a young player he still has a bright future no need to wait for him to move, just Chelsea find the right partner.
Chelsea know this player has potential so they have already secured his contract until 2033 which was extended since last August, it looks like Chelsea doesn't want Mbappe's case to happen again to this player. Chelsea also did the same thing to Nicolas Jackson securing players for the next 9 years.