TL;DR: Both sides have valid points. Simone Biles is a human being who may very well be struggling with mental health, however, she also paraded around less than a month ago about being the GOAT and proceeded to disappoint her team, and the world. I personally see both sides and am relatively neutral on this subject, and I won't attack you for whatever opinion you may hold.
My take as a former elite A-Level 6 Star recruit High School football player. Simone Biles is both right and wrong, and both sides of the argument have credit. Simone Biles is likely the greatest gymnast of all-time, and a big part of Team USA's success. She is also a human being. Mental health is not to be discounted, and if a person says that they need to take a break from something, or they need time alone to cope etc. that person should not be discounted. You don't know what somebody is going through, especially a world-class athlete with the expectations of their nation on their shoulders. HOWEVER, Simone Biles was parading around no less than a month ago about her being the GOAT of gymnastics (which is true, regardless of this incident), and making a big ruckus about her success. If you do shit like that, and then pull out of the final event and are likely the reason why your team got Silver instead of Gold, you should not be immune to criticism. Imagine if a guy like Jon Jones or Amanda Nunes went on a huge marketing campaign about their GOAT status, only to pull out of their next fight on the day it's scheduled to happen because of "mental health". They'd be crucified. Being a world-class athlete comes with tremendous mental stress and pressure to perform. That's part of it. GOATs step up to that. Simone Biles did not (at least in this case. There may be numerous other times where she felt this way before an event but decided to muster up the strength to push on, which is admirable). I am not going to call Simone Biles a pussy, a quitter, a liar, or any of that. But I will simply state that in recent years, mental health has become a crutch for people (usually women who have a minuscule fraction of the pressure faced by Simone Biles) to cling on to as an excuse for making questionable and bad decisions. Mental health struggles are not something that the person going through wants to publicize if they are as serious as they make it out to be. Granted, that goes for the common person, and not Simone Biles. There was no way Simone Biles could do what she needed to do for her battle with mental health without it getting publicized, she's an Olympian and that's not her fault. Regardless, both side's arguments hold water. And whether you agree or disagree with her decision, she is still a human being just like you and I.