I knew someone would ask this and it's difficult to point to exact parallels because H&M's fame is driven by the royal connection which gives it an entirely different texture to 'normal' fame (and explains why their fall from sublime to ridiculous has been particularly precipitous).
Fergie in the UK in the 90s would be one. Went from Royal It girl to laughing stock. She brought it back by staying quiet and focussing on clever commercial ventures in the US, having stayed out of the limelight for a bit.
I suspect that Diana might have gone the same way, had a tragic death and subsequent beatification not intervened.
I would say an interesting parallel study is the American rapper Nicki Minaj. Between 2008 and 2016 she was rightly lauded for her lyrical skill and verbal dexterity, as well as for being one of the few female rappers to hold her own in a particularly misogynistic musical genre. She had hit after hit, awards, the lot. But then she began making missteps: she married an unrepentent sex offender, had some dreadful plastic surgery, and her music sales began to fall naturally as she aged and a new generation of female rappers began to emerge.
It's how Minaj dealt with it that really flipped the 80-20 rule against her. Since 2018 she's been behaving in increasingly bitter and narcissistic ways, harrassing people online, playing the victim, planting fake stories in the press to heroise herself and denigrate others, and weaponising her hyper aggressive core of superstans against other rappers and rap fans on social media (leading to actual harrassment charges).
As a result, she's flipped the 80-20 rule. Her every move garners clicks, but it's mostly people laughing at her. She still has her ore of rabid fans, but they're not numerous or rich enough to sustain her musical career, so her album sales continue to decline and her tours fail to sell enough to return a profit.
@EdithWeston you make an important point. The Royal Family measures its PR trajectory in centuries. Whereas 'normal' celebs measure it in years, months and in the internet age, mere days and hours. It's often said that Meghan failed to understand the RF when she married into it; I personally think she understood it just fine from a quotidian perspective, BUT, crucially, she failed to understand that the RF measures time in an entirely different way to ordinary celebs. Meghan still thinks in terms of days, weeks and months - she failed to make the transition to thinking in terms of decades and centuries, and she failed to understand (as Kate grasped well) that the RF is a centuries long PR strategy, and much, much bigger than any single individual.