One early negative use of '[something]-shaped' is from Coleridge's letters:
1801 S. T. Coleridge Let. 31 Oct. (1956) II. 772 Old Symes—i.e. a Man-shaped animal capable of 'a deep malignant Rage'.
Earlier uses of '[something]-shaped' for invective certainly exist. For example, in The Gentleman's and London magazine of May 1756 (p. 265), the reviewer remarking the "List of Books Published" says, about A Satyrical Review of the Manifold Falshoods and Absurdities Hitherto Published, Concerning the Earthquake at Lisbon,
—This writer takes occasion from the various mistakes that have happened in the accounts of the late earthquake to abuse in more scurrilous terms than perhaps were ever printed before, all who have contributed to lay them before the publick. He calls them "dastardly mongrel insects, scribbling incendiaries, starveling savages, human-shaped tygers, senseless yelping curs, blushless caitiffs, common plunderers, ....
Early uses in invective are related to contemporary derogatory uses of the extended phrase '[something]-shaped object'.
Although the epithet was undoubtedly in use earlier, "piano-shaped object" and its initialism "PSO" was in use by Juilliard students prior to 1979:
...these [Juilliard] students have a pleasantly ironic way of talking (a "PSO," for example, is a "piano-shaped object," the kind of inferior instrument most pianists have to play on most of the time)....
— From a book review of Pianist's Progress (1979, Helen Ruth Ruttencutter) in The Boston Globe, Boston, Massachusetts, 30 Oct 1979, p. 40 (paywalled).
So derogatory use of '[something]-shaped object', accompanied by a suitable initialism, is student slang from more than 40 years ago.
Even earlier, and perhaps the source of the student slang at Juilliard, "piano-shaped object" (without the "PSO" initialism) shows up in a 1953 book by Lady Joy Petersen Packer, Apes and Ivory:
A few hours later he would be anxiously trying some "piano-shaped object" in Worcester, wondering, no doubt, whether he could hope to find it properly tuned and the notes in no way afflicted with rheumatism.