Like the public in general, critics have largely been disarmed by the art of Jeff Koons. To question his achievement is tantamount to attacking puppies, geraniums, and Cheez Whiz, all of which have figured prominently in his work over the past 20 years. Clearly one would need a heart of flint to attempt it. Let us begin.

Since 1987, one of Gotham’s more pleasant harbingers of warm weather has been the re-opening, in springtime, of the Metropolitan Museum’s Iris and B. Gerald Cantor Roof Garden. Over each of the past 11 seasons, a single sculptor has been invited to display his wares, and this year the honor goes to Mr. Koons. So charming is Central Park in the plenitude of springtime, and so majestic are the skylines of Midtown and the Upper West Side, that anything would look good against them. Although only three of Mr. Koons’s sculptures are on view, it is a fair guess that they look about as good as they ever will. But is that good enough?

Mr. Koons’s three sculptures, eclectic in form and monotonous in message, are confected from high chromium stainless steel with a transparent color coating. “Balloon Dog (Yellow)” (1994-2007) reverts to one of the artist’s oldest tricks, re-creating in gleaming, durable metal the flimsiness of balloon animals. Rising 10 feet at its tallest, it lingers somewhere between a child’s toys and a Macy’s Thanksgiving Day Parade float.

A second sculpture, “Sacred Heart (Red/Gold),” from the same period, consists of a similarly oversize Valentine’s Day bonbon, shaped like a chocolate heart and covered in what is supposed to be crinkly red and gold foil. The presumably subversive subtext here is conveyed more by the title than anything else, with its intimations of having something to say — without quite saying it, of course — about organized religion, as well as the multibillion-dollar confectionery business.

Finally, there is “Coloring Book” (1997-2005). The tallest work on view, this presents itself as an illegible mass of parti-colored metal. Only by reading the brochure provided by the museum do we learn that it is a slightly abstracted three-dimensional projection of a crudely drawn and monochrome Piglet (of “Winnie the Pooh” fame) taken from a coloring book. Over the schematic forms, Mr. Koons has splashed an array of colors with all the incontinent abandon of a 3-year-old who has gone off Ritalin.

As with most of Mr. Koons’s work, the manifest content of the three sculptures is one of shrill, slightly unbalanced exuberance. Underneath the dopey exteriors, however, we are assured that a more sinister message lurks, one that is intended to challenge our notions of art, religion, and the commercialization of both. Even if such a point were deemed worthy or interesting for visual artists to pursue, it is hard to see what power it could possibly retain at this late date, and in this specific application. Indeed, of all the sculptures displayed on the Met’s roof garden during the past 11 summers, it is difficult to recall any others that looked quite as wan and feeble, quite as toothless, as the three works by Jeff Koons.

Some critics — more eager to attack institutions than artists — will aver that the immensity of the sky and Midtown belittle Mr. Koons’s work into irrelevance. But this deficiency is a failure of the artist, rather than one of nature or of the urban fabric. If, in their nakedness beneath the solstitial sun, these three sculptures seem less powerful than some have remembered, that is because they had little to offer in the first place. In fact, they are ultimately a composite of four or five things we have seen often before: Duchampian ready-mades; the mining of popular culture in the Pop art of the 1960s and the Postmodernism of the 1980s; the play with scale in a way that was so essential to Minimalism and to sculptors like Claes Oldenburg, and finally, the presumption of engaged commentary on commodification, which, ironically, has been a big sell in the art world over the past generation. Yes, we have seen it all before, and from many a purer and abler priest than Mr. Koons.

And yet, it is a fair prediction that visitors to the roof garden will experience something that they mistake for aesthetic pleasure as they examine these three sculptures. It is a complicated mirage that begins with the recognition of the works in question. You have seen them — or works very much like them — often before on the international art fair circuit, in museums, and in the art press. And so they look like “classics,” like something that has always been there, like “Whistler’s Mother” or “Mona Lisa.”

Indissolubly joined to that apprehension is the knowledge, which has now lodged in most museumgoers as an aesthetic reflex, that the art is unbelievably expensive, that something like it recently sold somewhere or other for $10 million or $20 million.

In previous years, you had the impression that the Met invited sculptors to occupy its rooftop as a way of bringing them into slightly closer proximity to the pantheon of art history. And a number of the honorees have been equal to the encounter or have seemed to rise to the occasion. Regarding Mr. Koons, however, one senses the Met’s complicity in a dreary — and, we must hope, temporary — collapse of standards. It is as though the Met itself felt that it was being honored by the encounter, as though Mr. Koons, with his Chelsea street cred, were doing a favor for this straitlaced institution by aspersing it with the chrism of contemporary relevance. Paradoxically, neither party emerges the better for this transaction. The Met seems stodgier than ever, while Mr. Koons has been neutered and defanged by this uptown cooptation.