So I’m thinking, as Beth Ditto flashes past me again, a luminous blur of Dover sole-white skin, carmine lips and espresso-coloured hair, that, well, this is all getting a bit radical. Because this is about the tenth time I’ve seen Beth at a fashion show this season, in a whiz of flashbulbs, another bespoke designer outfit and in the front row, sacred pasture of the thin and must-get-thinner. She also, I notice on the sixth or seventh day (no Sabbath during fashion month), has a nut-brown stain on one of her creamy legs. It’s mesmerising in its imperfection, partly because imperfection is so rarely allowed to just be, in this collision world of celebrity and fashion. She’s so unself-conscious, it’s a moot point whether it could be classified as an imperfection, because isn’t imperfection amplified or diminished according to how the person with the alleged imperfection deals with it?

Could it be that in 2009, with everything else turning upside down, fashion has finally smashed its only real taboo? Has fashion really gone and made a woman who’s at least size 20 its mascot? And if so, is this the start of something genuinely new and important?

Let’s begin with the positive. Beth Ditto is fashion’s mascot this nano-second. Since Christopher Kane made one of his bodycon dresses for her a couple of years ago, designers have woken up to the thrill, the drama, the sheer kinky frisson of taking their Lilliputian creations and sizing them up. Fashion loves extremes. That’s what makes it fashion rather than simply clothes. And she’s extreme.

She’s also on the cover of Love, the new magazine – naked. She’s at the Stella McCartney show, the Loewe show… laughing raucously, and not in that decorous way that celebrities tilt their heads and half open their lips to indicate merriment, but a full-bottled head toss – that’s a breach of conduct in itself. She’s like a ripe, soft cheese oozing over its wrapping. She was the star turn at Fendi’s party in Paris, with her band the Gossip (last year Amy Winehouse provided the cabaret at the same gig). This is a girl who’s excessive in all the ways that fashion likes to be freakily controlled – and she’s being celebrated for it. Forget showing models smoking (although French Vogue apparently has a forthcoming shoot in which a model sports a prosthetic pregnancy belly and cigarettes), putting a fat girl at the heart of fashion… that’s daring.

Not that it will make a jot of difference to the clothes, or the sizes. Designers will continue to stop at size 14 (some at size 12) because that’s how it works. And big women will continue to feel alienated by fashion. Beth Ditto can work a bodycon dress because she appears to suffer from about as much self-doubt as a tornado. You might find the way she looks too in your face. You might argue that she’s no more a healthy role model than a girl who’s too thin. But you can’t argue with her confidence. Maybe in the end, this is about attitude, not shape. That’s radical too.

Times recommends

• McDonald’s: the world’s local restaurant
• Help me to be a mum: a foyer to fulfilment
• A life online: God likes a laugh