https://stats.nba.com/league/player/#!/playtype/post-up/?dir=-1&sort=PlayerFirstName
So people have actual stats, not just "Remember that one time he got blocked? he sucks at posting up".
According to the stats, he is the 52.7% percentile on post-up efficiency. So he is a very average post player, but he hardly ever post-up (he actually has the 14th lowest post-up frequency of his possessions. The only SF with a lower frequency is Otto Porter (Aminu is tied with Hayward) I just don't think it's a very reliable thing in Utah's offense with Favors/Gobert. Maybe it's something he could get better with if he plays with Lyles more.
The one clear positive about Hayward's post up game is that he actually does draw FT trips out of his post-ups at a high rate compared to the rest of the league and he turns the ball over at an extremely low rate compared to the rest of the league out of his post-ups.