His second season was poor. But to put it in context, Mourinho came in and quite clearly took a dislike to him, played him in a new position (he'd previously played as a striker or wide attacker, but Mourinho wanted him more as a winger coming deeper to help defensively), plus Martial was having personal problems off the field, all at the age of 20. So yes he was poor, but hardly a surprise. Even so, his performances were improving towards the end of the season.
His third season wasn't poor, he was quite clearly our best option on the left that season (and for a month or two he was our best attacker fullstop) but Mourinho kept looking for any excuse to drop him. Kept dropping him for Rashford for the first half of the season despite Martial playing far better, then instantly moved him to the right wing (where Martial was never able to perform) and dropping him once we signed Sanchez.
If we look at his first five seasons as a whole before the injuries really started, he had two clearly good seasons, two seasons where he was fairly good when he was actually given a run of games but Mourinho did everything he could not to give him that, and one poor season. With his best season being the last of those five, it was a fairly normal and expected progression for a player who only turned 24 during that season.
It was only in 20/21 that it all went to shit.