If you think Drummond is a shit defender, then Zion is the worst defender in the league. Drummond isn't a shit defender, he's been a traditional 5 in bad Piston teams. Yet his PER, defensive win shares, and defensive rating per Basketball Reference is very impressive.
As I said, right now Zion has no jump shot so he can only score points in the paint and usually at the rim (like Drummond). He's one of the best finishers at the rim.
But at only 6'7", he has trouble defending bigs and if he's asked to defend a floor spacer, he gets lost in space way too often and cannot defend any pick and roll sequence effectively throughout an entire game.
Zion is still learning the NBA game, but he has to develop so much more and that includes a legitimate midrange jumper and be a reliable defensive presence on his team.