If anyone deserves to have a show at the National Portrait Gallery dedicated to photographs of their face, it’s beautiful, doe-eyed Audrey Hepburn, right? If anyone deserves to have their exhibition subtitled ‘Portraits of an Icon’, it’s Audrey Hepburn, right? She has become an icon: evoked in fashion editorials for her ‘timeless’ and ‘classic’ style; that Breakfast at Tiffany’s poster a student halls cliche; her gamine presence even posthumously inserted into adverts for chocolate. This show of 150 pictures of her neat, sweet face, her pretty pastel frocks and signature Little Black Dresses is bound to be a ticket-shifter.

I wish it wasn’t though. I mean, yawn. After gallery shows on Grace Kelly and Marilyn Monroe, I’m pretty bored of this fetishisation of the image of woman – and it is, make no mistake, all about the image. This isn’t a show about Hepburn’s talent, her brains, her personality – or how clever photography might open up those qualities or reveal something new about her. It’s about how she looks: the one thing we all already know.

Icon is the word is used either for a devotional religious image or for ‘a person regarded as a representative symbol’. Both of which seem pretty apt. The drooling here – as everywhere; the show is responding to, not creating, an iconography of Hepburn to be fair – is over her grace, her composure, her ingenue poise. She’s non-threatening, but still – yuck-word alert – aspirational. She’s slim, with perfect, precise hair and make-up, and often draped in jewels. Glamorous without being dangerous. Even her most famous role after all, in Breakfast at Tiffany’s, was a much tidied-up portrayal of a chaotic female character.

Yes, yes, her films are charming (and sometimes edgier than her star image), she’s gorgeously watchable, a significant screen talent… and anyway, we all need a bit of beauty or escapism in our lives now and then. Judging by the coos and squeals of visitors to the National Portrait Gallery – from women young and old – the appetite is there all right.

And this show does introduce images of Hepburn’s committed volunteer work with Unicef, which is obviously A Good Thing. Or there are snaps showing Hepburn in slightly surreal Sixties fashions or off-duty in the countryside, holding a baby donkey or a little lamb. But they’re still very staged; she still always does coy eyes, glancing over her shoulder or peeking out from under those thick eyelashes. These are stylish professional pictures by fashion and art photographers, including Cecil Beaton, Terry O’Neill and Irving Penn, so of course they want her to look good – but a striking shot isn’t the same as a flawless one.

It’s all just so damn elegant and demure, whether she’s in chic black cropped trousers and ballet pumps or floating on a cloud of tulle couture skirt. The enduring obsession is with Hepburn as a model of femininity, a creature of grace and lightness. It me feel restless and rebellious. I want to reach into those portraits, and rip a ladder in her stockings.

I don’t want perfection. I don’t want decorum and polish. I don’t want great icons of femininity to be just about the ‘right’ appearance – slim, young, white, rich, pretty, neat, just to use the objective adjectives. Being kind to children and animals.

I want venerated womanhood to be a bit fucking messy. I want to see her laughing unexpectedly, with friends, with her mouth right open. I want to see her with a stonking hangover and egg stains down last night’s ballgown. I want to see her in mismatched wooly socks on the sofa, completely focused on a filmscript. I want to see her angry at a political demo.

Maybe I’m being deeply unfair. It’s probably not her fault there are no pictures of her staggering home at dawn with a nicotine moustache from chaining too many Lambert and Butler during a night on the voddy – I daresay Hepburn didn’t engage in such activity (and if she had, she’d probably have had that Breakfast at Tiffany’s long cigarette holder elegantly to hand). And not all of our high-profile women need to be badly behaved – some are, and they end up suffering fetishisation of another kind.

People will say she wasn’t about any of that. That she carefully stage-managed her look: old-fashioned, put-together grace and glamour, a product of her time and worth celebrating as such. But I still can’t help wishing that, in our own time, our feminine icons could be, well, a bit less saintly.