Normally, I'd agree with you that mental health isn't really a low blow as far as things go, but two things exist that change the context of this:
1) Drake's a dude who's made millions wallowing in his feelings. Like, legit millions. He's made bank talking about that feeling when you break up with your ex, or that feeling when you love her and she don't, or that feeling when you and a girl connect and then never meet again. And now all of a sudden, he wants to act hard and chastise a man for publicly spilling his feelings in the open. Nah, that's not how it works.
2) Cudi's declaration changed the goddamn narrative. He's no longer some cocky dick, he's a dude struggling with mental illness and suicidal thoughts, who had the balls to tell his fans his public struggle and help people open up about their issues with it. He's damn near untouchable now as he's an icon for more freely talking about issues rarely talked about due to masculine stereotypes. And this is the dude Drake takes a shot at, taking the very positive thing Cudi's done in the eyes of the public and using it as a weakness of his.
In any other situation, Drake's shot would be alright. But, Cudi's declaration fucked it all up.