(Some) Devils fans hate him because 1) he's not Rantanen, Barzal etc 2) His low effort and lack of interest at times.
But he's not negative value at all. At 3m he is a decent 0.5ppg middle6 who is best at wing but can fill in at center incase of injuries. He was devils leading scorer just a year ago (doesn't mean much but still).
He's not bad as much as he's frustrating, Devils fans have waited years for him to become solid top6 player and have grown tired of waiting. Initially many said they'd be ok with him if he becomes a 0.5ppg player, which he has been for the last 3 years, but the same people still want him gone.
Edit: Just wanna add that there are plenty of worse players in the Devils lineup like Johnsson, Kuokkanen, McLeod, Bastian possibly even Tatar. But none of them get as much hate as Zacha.