Put the poem into ordinary word order and its trick shows: "Our one pretty pear for supper is as sorry as a world with no art." That is a comic comparison. A thin meal is measured against a cultural wasteland, and the poem gives the huge thing first and the tiny thing last. The reader hears about the barren world for three lines, and only then does the pear arrive, set down like a single object on an empty table.
The first word, "sorry," does double duty. It means pitiful, as in "a sorry sight," but it is also what a host says before serving something inadequate: sorry, it's only a pear. The title adds a third layer. "Pear-shaped" is a literal description of the fruit, and it is also a British idiom for things going wrong. The supper has gone pear-shaped in the most literal way possible, because it is nothing but pear. The pun is deadpan, and the whole poem keeps that straight face.
The construction supports the joke. There are no capitals and no punctuation, so the poem sounds like an offhand remark overheard at a kitchen table rather than a composed statement. The first three lines each have four words, and the last, "for supper," has only two. The poem shrinks as it ends, as if the meal were being cut down to size while we read. The word "our" also matters. Someone is sharing this one pear with someone else, which suggests a small household and a small domestic disappointment, and it makes the grand comparison to a world without art feel affectionate rather than bitter.
The line break after "is" adds a second reading. For a moment the sentence can run as "a world with no art is our one pretty pear." In that beat the pear becomes the entire world, stripped of everything else, with itself as its only decoration. Then "for supper" pulls the sentence back to the table, and the pear is food again. The poem keeps tipping between the two.
The word "pretty" complicates the comparison. Someone describing a meagre meal might say "ripe," "hard" or "lonely." This speaker says "pretty," which is a word for looking, not eating. The speaker is admiring the pear's shape and colour, and a pear is a classic subject for painters of still lifes. So the thing being called as bleak as a world without art is itself faintly a work of art. The poem complains and consoles at once. The supper is sorry if what you want is dinner, but not if what you want is something to look at. Perhaps that is why the world in the poem is never wholly without art while the pear sits there.
The sounds work quietly in the background. There is no rhyme, which suits a poem about lack. But "sorry" and "supper" open and close the poem on a similar frame of sounds, an s at the start and an r near the end of each word, so the poem curls back toward where it began. The p sounds gather in "pretty," "pear" and "supper," as if the fruit were being said three times. "Art" and "pear" also share two of their letters, and the near-echo makes them seem like relatives. The poem never points to the link, and the reader has to notice it.
The poem matters because it fits so much into so few words. It treats food and art as two kinds of nourishment, and it suggests that someone who measures a skimpy dinner by the loss of art has already shown what they value. It does this without any speech about the importance of culture, and a joke carries the whole argument. The reader does the work of seeing how a pear can be both a poor meal and a small piece of beauty, and that small act of noticing is what the poem is about.