Why is that? Genuine question
And it's always the ones without any indicators of a diagnosis
The most hated ones are also the ones with actual signs of conditions or illness LOL
Like Abuela Alma in Encanto who has clear undealt-with trauma. No one has sympathy for her but for some reason every other family member has some sort of condition cause that's what the fandom says 
Another one that comes to mind is Elsa. She has notable isolation issues and unstable and impulsive emotions but she's called bland, "terribly-written", or too irrational...... That's the point!!!! 
For me the worst is the crap Hannah Baker gets for being bad represention for depression cause she's "stoopid" and "always contradicts herself". E.g. when she asked her bf to go away and later said that he abandoned her, when the principal offered his help but she refused it, etc.
But the fact that they're so irrational is actually really accurate because a nentally ill person acts and thinks without logic, because the ill brain works outside of the norm. It's called a mental illness because it affects your daily life through behaviors and actions
Isn't it pretty wrong to say that a character is bad represention of mental illness because they are irrational or act outside of the norm? 
or that they're a bad person because they act and speak without thinking straight?
Why would you want a character representing depression to be written like a person without depression? 
I assure you most of these fans diagnosing other people with conditions and justifying it by saying they have one themselves have perfectly stable brains and are doing it cause it's a trend. The healthy brain will never understand the ill brain so we need to stop acting like doctors or diagnosed patients 
