Dirs: Abby Kohn, Marc Silverstein. Cast: Amy Schumer, Michelle Williams, Busy Philipps, Naomi Campbell, Aidy Bryant, Emily Ratajkowski. 12A cert, 110 mins

Amy Schumer doesn’t need too many excuses to be funny, and she certainly doesn’t need the trumped-up self-body-shaming of I Feel Pretty, her new vehicle as a producer and star. The film wants it both ways: to reject the unattainable hierarchies of the beauty industry, but also to ridicule a person who doesn’t know her place within them.

The only means it can find to be funny is sabotaging its own message, which isn’t a great starting point, let alone finishing point, for a body-positive comedy.

Renée (Schumer) is a minor employee at a scarily top-end New York cosmetics firm. She’s as far from front-of-house as could be imagined, sifting through web orders in a dingy basement that's miles from the gleaming HQ uptown. And she hates the way she looks, trapped in a state of precarious mental health which can’t be helped by her choice of employer.

Brought to mind in a scene when she watches Big (1988) are the high-concept comedies of the 1980s. One of these might have decided to port Renée into someone else’s physique – say, the lissom style princess played here by Michelle Williams – and watched them grapple with their change in fortunes.

I Feel Pretty is much more specifically about Renée’s mental hang-ups, so it puts the equivalent situation inside her head, engineering what you might call a psychosomatic body swap. She gets a clunk on the skull during cycle class, and becomes convinced she has been magically transformed into a total babe.

All the confidence she lacked pre-concussion suddenly floods from her in embarrassing waves. It’s the mismatch we’re meant to find funny – Schumer wafting around regally like she’s Taylor Swift – but it’s nothing doing. Other characters, most of them the official megababes she had always found daunting as colleagues, stutter and gawp at her lack of self-awareness.

Auditioning to become the company’s new receptionist, she finds herself in front of the Williams character and Naomi Campbell, who try hard to pretend they’re not judging a particularly painful routine on RuPaul’s Drag Race. But there’s only so much eye-widening from the supporting cast one film can use as an all-purpose punchline, without seeming cheap, cruel and self-defeating.

Michelle Williams

This is the first feature from a pair of screenwriters, Abby Kohn and Marc Silverstein, who have tended to specialise in variations on the ugly duckling myth. The titles – Never Been Kissed, He’s Just Not That Into You – speak for themselves. But I Feel Pretty doesn’t manage an honest internal transformation of any kind. The scales fall from Renée’s eyes only after an hour of unwitting humiliation, and in the hastiest way the film can think up – another concussion undoing the first one, and lessons swiftly learned.

Schumer can’t solve this shedload of problems all by herself, but at least she can busy herself pretending nothing’s amiss: easily the best scenes let her openly improvise with whoever else happens to be on screen.

It’s a waste, too, of Williams, who has a lot of bright ideas for her role – delivering her lines with a helplessly unauthoritative poo-poo-pee-doo vocal tone, and voguing ethereally as if made entirely of perfume. She’s the funniest thing in this, actually.

The character, one Avery St Clair, is mercifully free of malice, because she’s so caught up in her own tense consciousness of selling a dream that she barely notices how far anyone else is falling short. The film might throw Renée repeatedly under a bus, but it's some relief that this ever-smiling Boss Barbie never does.