Martial went from the focal point of the team, top goalscorer with 2600 mins as a 18/19 year old under LVG in 15/16, to averaging ~1000 mins less, a bit part player, shunted out on the wing under Mourinho for the past 3 years. 3 years is huge. I don't care who they are, that is going to significantly affect any young player's development. It's not like he has been a key player throughout his time here.
This is the first year since, he has again, become a focal point of the team. He's already surpassed his PL minutes for the whole of last season, (1638 vs 1623). There has been clear improvement in his stats from last season (9G+3A in 20 PL games vs 10G+3A in PL 27 games).
I don't know where the stereotype of him being stupid comes from? When next to top talent, he has never had a problem being on the same wave-length. Anyone watching our matches can see, he has been the one constant, providing the clever movement and linking the attack together, for the rest of the attack to thrive.
Also, he is only 24. Your list of top strikers that highlighted where they were at the same age is true. People need to have patience.