In the past I think there were fewer celebrities that wanted to be 'fashion icons' so the truly stylish ones were the ones that stood out (Bianca Jagger, Sophia Loren, Audrey Hepburn etc). These days everyone wants to be seen as fashionable for two reasons:
1) It makes sense money-wise as fashion is now big business and if a celeb is to be offered a fashion line, they need to show an interest in fashion
2) Paps are everywhere so the more you're seen to rock a trend, the more you'll be photographed. In London, the likes of 'cool dressers' like Agyness, Alexa, Daisy Lowe, Kate Moss and Pixie Geldof are in the tabloids much more than A-listers like Madonna or Gwyneth. Celebs have realised that if they dress 'fashionably', they'll get photographed more and raise their profile.
The main issue is that *those* celebs then get known more for their style than their original talent. It kind of dilutes what they're really known for.
Celebs have ruined fashion for me as there are so few really innovative ones. As someone already said, they all have stylists so they're all more or less wearing the same designers and trends. BORING! In the past, celebrities didn't have stylists for every day so their own innate style came through.
On the other hand, it's because celebs got involved that fashion became so popular. And if fashion is more popular it means more magazines, websites, blogs and jobs for us fashion types! And also, by having non-classically beautiful celebs on covers maybe it helps young people feel good about how they look when they see this diversity? (Ok, the airbrushing/size zero debate comes into play but I'm just touching on the subject.) That said, I still think
covers belong to models. Back off celebs!