I agree with what I think you're saying. Cartoon villainy (regardless of whether or not it's Sigma's own undoing) is kind of separate from real-world issues, and I don't believe Blizzard should have to tick certain boxes when designing every character. They have one of the more diverse rosters out there, even if most of the details are outside the game. Personally I love the angle of Sigma's backstory, but I'd be interested to hear from someone for whom this hits closer to home, and if/how they identify with the character.Sigma's mental health problem seems to be "consciousness split across time which makes him experience events in a nonlinear fashion" ,which really isn't the sort of thing that you put in the game to add diversity/representation. His backstory is straight up Event Horizon-style cosmic horror.
That Sigma's a victim being used and abused by others taking advantage of his current state seems to have been a deliberate choice to avoid the Spider-Man style "lab accident causes good scientist to go evil" trope.
Also, maybe I've got my definitions mixed up, but are you confusing trope with cliche? It seems to me that they're straight up using the "lab accident" trope (nothing wrong with this in itself), while attempting to subvert the "good scientist gone bad" cliche. Either way, I think they're successful thanks to the exploitation angle and the sheer mind-boggling nature of his condition/abilities.