Just the idea of "prison poets" has a number of very funny possibilities.

But devoid of context, the poem isn't very funny.

snipe

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‘Lay off those pink Gins, and while you’re at it cut out the starchy foods and take regular daily exercises, because a girl who marries a man who looks like you do at the moment of going to press is going to have the uneasy feeling that she’s committing bigamy.’
--P.G. Wodehouse, “Pigs Have Wings"