I write as someone who lived in South Kensington in the late '70s. I was born a year earlier than Princess Diana, and used (not knowingly) the same hairdresser. I do think she was a fashion icon because of her media presence, but as a women aged 18/19 at the end of the 70s I would never have dressed as a Sloane Ranger with the piecrust blouses, etc. Punks were everywhere in London at that time.
I don't think Diana was ever a trendsetter, or paved a way for other women, but what she did have was charisma, and she used that for the benefit of others.