As I recall part of the problem with Diana's dress was that she kept losing weight, far past the point where they could take it in any further. Another problem is that the design was simply awful, although the length of the train was deliberate because of the size of St. Paul's. The designers were newbies, which did show in the overall effect. The choice of material was tragic, because they could not stop the wrinkles.
Mostly, though, I think that like the rest of us, Diana was much younger (19) and had the same unformed taste that the rest of us had.
I have always wondered if her floppy hair style was a deliberate choice, to go with the sleeves.