Faux feminisim: Is comtemporary art paying too much attention to work that should be ignored?

"When I am an old woman," begins one of the nation's favourite poems, "I shall wear purple/ with a red hat that doesn't go, and doesn't suit me." And it goes on in that vein, listing an assortment of charming but harmless eccentricities, quite as if they were the last word in senior troublemaking. But how about this: when I am an old woman, I shall stand stark naked, legs akimbo, with a desperate look and a pistol in each hand, one pointed at the viewer, the other at my own head.

As you go through the glass door into the first room at the Serpentine Gallery, that is the picture that meets you. It's a kind of self-portrait by the Austrian painter Maria Lassnig (right), and it's called You or Me. What's disarming about it is that, for all its frantic self-exposure and viewer-confrontation, it's painted in a manner that can only be called friendly: neither neat nor wild but relaxed and lively, not too far from the later, looser style of David Hockney. Another surprise is that the artist is nearly 90 years old. One more is the venue itself.

Lassnig is a discovery. After a long career, and with some fame in other places, she is pretty well unknown to the British art public, including me. The work in the show all comes from the past few years – what happened before that, I am very vague about – but here it is all human bodies, or quasi-human quasi-bodies, in a repertoire of pleasure-pain-desire-frustration. "Body-awareness painting", the artist has called it, and it encompasses realistic figures and awkward, animated body parts with eyes. There is generally a comic edge.

It was exactly 20 years ago that the Serpentine Gallery "discovered" Paula Rego, and Rego is an artist whose work has some affinity with Lassnig's. They share a deeply ambiguous feminism, which rather revels in female victimisation and male monstrousness, and finds awful things funny. But the Serpentine today is under very different management.

Its agenda is cutting-edge. It shows new painting, but the kind of painting it shows – John Currin, Glenn Brown – is usually riddled with knowingness. It's not in the habit nowadays of displaying, let alone discovering, a perfectly heartfelt expressionistic figurative painter such as Lassnig. So where's the twist? Or at any rate, where does the gallery think the twist is?

I can't quite put my finger on it. Lassnig on this showing is a perfectly acceptable painter: fluent in her brush-drawing, vivid in her colour. She occasionally has a good idea for a subject, as in that dramatic self-portrait, or a series of images of people wrapped in crinkly polythene, where the bright interrupting slashes of reflected light push her broad technique to the limit of coherence. But, in terms of invention, accomplishment or general body-awareness, Lassnig doesn't strike me as being in a different league from quite a lot of figurative painters, living or lately dead, who certainly wouldn't get a show at this gallery.

My surmise is that some boxes are being ticked. Lassnig is old. She's a woman. She is sexually and psychologically frank, sometimes "troubling" – a brutish fat man has his way with a slip of a girl or a doll. She clearly still has her wits about her, and her painterly flair. And this adds up, I guess, in somebody's estimation, to an interestingly dissonant, feisty combination. (For all I know, she wears purple, too.) But what it demonstrates really is that the art world is in a terrific fizz about painting at the moment. It has suddenly decided that painting is not dead any more but very much alive. And like somebody startled from sleep, it can't quite tell the difference between anything.

That capacity will no doubt slowly return. In the meantime, confusion reigns, and galleries such as the Serpentine will promote painters such as Lassnig as an "avant-garde pioneer", while praising her for doing things ("depicting... inner sensations") that painters have been doing for ages.

I mean, what was that guy called? It'll come back to me eventually. Pablo Something-or-other.

Maria Lassnig, Serpentine Gallery, London W2 (020-7402 6075), to 8 June