There is a genetic component to mental illness but much of it can also be a response to stressful environmental issues. For example, Diana's mother who allegedly was not treated well by her husband, had problems with alcohol, but having lost a bitter custody battle over their four DC, and having lost a 10 month old baby, perhaps her issues were an understandable response to the situation she found herself in?
Diana admitted to having had mh issues when, in her very early twenties, and having had two small DC, it gradually dawned on her that her husband never loved her and never would. And that she was basically a pawn in machinations of the state and had been used to produce an heir and a spare and she was meant to keep quiet in public and pretend all was well and her DC were supposed to collude in the lie too.
Given that Diana seemed to blossom and become much stronger and confident after her divorce, it's fairly reasonable to assume that many of her mh difficulties arose out of the situation she found herself in. And how many of us would feel anxious, paranoid and upset if photographers followed our every step and mis-step, intruded on the lives of our DC, searched through our bins, chased us by car, paid money to our cleaners, family and friends, in exchange for stories about us? I think Diana did very well in very difficult circumstances and remember that Martin Bashir went to the extent of forging documents which fuelled her distrust, and Charles's friends briefed about her being mentally ill and are still doing so now!
Rather than people being labelled as mentally "odd" therefore, surely it's more accurate to say that people have periodic mh problems when they find themselves in untenable situations. Frankly I find it disappointing that on Mumsnet in 2022 in the UK, where 1 in 4 people experience a mental health problem of some kind annually, that posters blithely talk about mh issues in such an ignorant and regressive way.