Andy Warhol: Other Voices, Other Rooms, Hayward Gallery, London

I'm sure there must be a harder film to watch than Andy Warhol's Mrs Warhol, although I can't instantly think what it might be. Made in 1966, the work – at an hour and seven minutes, pretty well feature-length – shows the artist's 73-year-old mother in the dank basement she occupied in her son's Lexington Avenue house. Babbling away in a Mitteleuropa accent – she was born in Ruthenia – Julia Warhola plays the part of a much-married ex-movie star. Actually, Warhol's mum is just a senile old bat who irons, drifts between past and present and strokes the hair of her Andek's current pretty boy, Richard Rheem.

If, as her son said, every modern person will be famous for 15 minutes, then this is Mrs Warhola's quarter-hour. Warhol claimed to love America because anyone could drink Coca-Cola – the president, Liz Taylor, the bum on the corner – and it would always and only be the same Coke. Here, he hinted, was pure democracy, although it was the democratisation of junk. Likewise, fame is turned to rubbish by ubiquity. Encoded in Mrs Warhol is a deeply unfilial message: although she'll never be ready for her close-up, even this old bag can be Norma Desmond.

It is a cruel film, exploiting Mrs Warhola's poverty and accent but also Rheem's New York air-headedness. ("I cutted gress for cow eatink," recalls Julia at one moment. "You cut grass for cows?" Rheems squeals.) More than that, it is a dishonest film. Like all the 19 movies in the Hayward Gallery's show, Other Voices, Other Rooms, this one is shot with a static camera before which its stars ad lib unedited. Most of Warhol's 16mm films end when the reel runs out; often, you can see the mechanics of their making – a hand-held mike here, or a lighting boom or, in the case of Horse, the entire rig. Implied in this is a sense that there is no auteur, that Warhol isn't responsible for what happens on screen; that – as with fame and Cokes – everything is equal before the one-eyed god of film.

Although it sounds optimistic, Warhol's assertion that anyone can be a star is, in the end, nihilistic. His view of the world reminds me of the joke about the recorded instructions on a mental hospital switchboard: "If you are a clinical depressive, press any number you like. It won't make any difference anyway." And yet what makes Mrs Warhol so mesmerising is the looming absence of its director.

Warhol was self-hating, believing himself ugly, proletarian and unlovable. Far from being the impersonal thing it affects to be, this film is an anti-self-portrait in which Rheem is everything its director felt himself not to be: beautiful, the son of a star, loved by his mother. It is usual to say that Warhol's genius lay in anticipating the dumbing-down of the media that would end up as Big Brother. Actually, the films in this show – My Hustler, Blow Job, Sleep and the rest – are auto-biographies of a man who couldn't stand looking at himself.

It is Warhol's personal collision with the zeitgeist that makes him great, and made him give up painting for film in the mid-1960s. His camera-based works of 1963-68 – movies, polaroids, TV shows – are the subject of Other Voices, Other Rooms, although they are really in the Hayward as a kind of extended footnote, an explanation of the car crashes, the Jackies and Marilyns, the cans of Campbell's.

When she could afford to, Julia Warhola had fed her art student son a daily tin of soup for lunch, a different flavour every day. Contained in this is the crux of Warhol's world view: if anyone can afford luxury – clam chowder! chicken gumbo! – then it ceases to be luxurious. If everything is filmable – the Empire State building, a kitchenette, pretty young men, ugly old women – then nothing is. One of the clever things about Other Voices, Other Rooms is the way it feels cobbled together, so that everything appears equivalent. Film monitors on the wall are no different from the Warhol-designed cow wallpaper behind them; the soundtracks of Warhol's endless television shows babble into each other. I've long felt that Andy Warhol was one of the great moralists of the late 20th century, and the Hayward's show backs this up. Alas, the moral he repeated so often was "So what?", which does not make for cheery watching.

Hayward Gallery, London SE1 (0871 663 2501) to 18 January 2009