However I find it equally distasteful to read some of the comments on here about how plain she was
I haven't read any of those, but I agree with what Hilary Mantel said about her in the piece I already linked, when she imagines the safer and more ordinary (and probably much longer) life she would have had if she'd married someone else:
What would have happened to Diana if she had made the sort of marriage her friends made? You can picture her stabled in the shires with a husband untroubled by brains: furnishing a cold house with good pieces, skiing annually, hosting shoots, stuffing the children off to board: spending more on replenishing the ancestral linen cupboard than on her own back. With not too much face-paint, jacket sleeves too short for her long arms, vital organs shielded by a stout bag bought at a country show, she would have ossified into convention; no one would have suspected her of being a beauty.
www.theguardian.com/books/2017/aug/26/the-princess-myth-hilary-mantel-on-diana
I think this is right, rather than mean-spirited. You can see that she did, in fact, become a 'beauty', but her appearance is all entangled with her becoming the most famous woman in the world, and her status (especially after the Camilla Parker Bowles affair became public knowledge and the marriage ended, and she was cut adrift from the royal family) became that of a sort of sacrificial victim, and after she died, a glamorous dead icon like Marilyn Monroe or Princess Grace or Anne Boleyn. I mean, I don't think it's accidental that she was bulimic (she's so cruel in those odd tapes from her voice coach about what she looked like when she was younger she calls her young self 'the fat Sloane Ranger' more than once), or that her relationship with the paparazzi wasn't entirely one-sided, or that she was intensely conscious of her effect. Those were her weapons after the marriage ended, and she was no longer a future queen, and she embraced charity work and she won. (Could Charles have married Camilla if Diana had lived?)
Though I agree that if she'd lived, the shine would have rubbed off, and was already rubbing off. A perception was growing of her as just another dimwit celeb looking for a replacement husband (and where do you go to top a future king even if he made you miserable?), and her glamour couldn't have been preserved in the age of social media, where anyone with a phone is a paparazzo.