I think you're being to harsh about the sci fi book post, what the poster was obviously saying was that: "you wouldn't turn away a book solely for having a mentally ill, bisexual female protagonist.". Which is an issue, like in the movie version of Crazy Rich Asians, the author revealed they had tried to cast a white person as the main character. Some companies think that audiences won't respond to a non-white protagonist.
Also, sorry if that came out rude, I really didn't mean to say it in a rude way.
The fact is that making facets of a character the important parts or selling point of the character is lazy writing and overall bad development but no one is saying “don’t write characters like that. One of the most badass women in film is Ellen Ripley played by the amazing and versatile Sigourney Weaver but you know what really made her character great? She wasn’t originally written as a woman, but the casting decision was because the actress fit the overall characterisation of the role. Of course casting in media is problematic because it calls for a certain type of person based on race, age, body type, and a very general personality description, so I can agree that should be changed, but that’s not entirely something to compare to written fictional characters, which is what the post is talking about.
Personally I’d love to see a disabled bisexual character no matter what kind of character, protag or antag or anti whatever, and no matter the race or gender or if they are cis, trans, or non-binary, but I do not want to see those parts of the character being made the most important part of who they are or even the reason I should read the book. That’s the point of representation, not just that it’s present but that’s it’s honest and not done just for the sake of but naturally there.