It's important to realize that Charlotte wasn't a good person, however this doesn't make her a bad person, a villain or anything of the sort. Let me explain.
Throughout the show, Charlotte manipulates narratives so she can seem like the victim and everyone else is wrong. Examples:
When she told her group about her injury, she left out that there WAS a sign indicating that there was mold, she acted recklessly and got infected.
When Parker called her out on spending the money that was meant to go for her surgery on other things, she began saying how it was their fault when she specifically said she didn't want it.
It's implied by that same conversation that this is the reason most of their friends are distant with Charlotte, because it wasn't the first time she pulled a stunt like this.
She deflects the situation to make it look like she was never at fault, but it was other people who should have known what was going to happen and she was the victim.
Now, with this said, she does show signs of being a really nice person and fun to hung around. Her relationship with Circle, although not expanded in the show, did imply they got relatively close. Parker himself cares deeply about her, which begs the next question:
What happened to Charlotte? What changed?
Of course, one can only speculate. Bad parents, awful work environment, terrible friends who hurt her in the past... Whatever affected her clearly had a big impact on her.
First, she always seems to have a point to prove. Charlotte has developed this tough persona through the years and is important for her to keep it. She denies herself getting help from her friends and has a hard time saying what she feels (hence, bad temper as a form of protection).
Charlotte also shows signs of self sabotaging every attempt at getting better or living a healthy life. She likes high-risk sports and the mold is clearly killing her, yet refuses to get it treated and doesn't seem to care about it (again, back to protecting herself of showing vulnerability).
What we see in the show is a person who is struggling and clearly needing help, but denies it and hurts the people she cares for in the process without wanting it.
Once she comes back from The Waiting Room after Texty accidentally killed her, this event seemed to have finally clicked something within her. She finally opens up to Amelia, voices up her concerns and makes a genuine attempt at wanting to connect with others, the implication being she wants to get better after realizing what her future could be if she doesn't treat the mold and isolates herself.
Going back to my original argument, yes, Charlotte is not a good person, but she is aware of how her actions have negatively affected others and how this made her be perceived as a bad person. The Plane is a life changing experience for her and, if it wasn't for the open ending, she's probably ready to seek out the help she needs.
