I'm somewhat reluctant to re-open this can of worms, however one can argue that the time when Carolyn was popular was when the whole Calvin Klein minimalist look was in (and yes, she certainly helped promote the look). Additionally she was involved with Calvin Klein, and that obviously influenced her aesthetic, and due to her position of being the wife of JFK Jnr I would propose that it would have been socially more acceptable for her to dress in more classic, understated looks.
Lauren Santo Domingo on the other hand works at Vogue where she would be exposed to so many different trends, and has not married a scion of a 'great American political family' so does not have any societal requirements of dressing in the same way. Whilst I've been a harsh critic on her constant trend-hopping, I don't think it necessarily shows that she is somewhat inferior status/class/fashion-wise. She tries 'harder' in the sense of more "wow, look how I can pair my Proenza pieces with new season Phi!!" to appeal to the 'fashion' crowd rather than the traditional 'high society' crowd.
Additionally, Lauren has also worked as a model (I'm pretty sure she once said that she once modelled alongside Brad Pitt in a jeans ad - I can't find this image online though!)
To me, Grace Kelly, CBK and Lauren are all just so different it never even occurred to me to compare them with each other.