Fig.: said of something extremely ridiculous or ironic. 
(informal)

Are you serious? Are you really going to wear that ridiculous outfit at the gala dinner? It's enough to make a cat laugh!

Fai sul serio? Hai davvero intenzione di indossare quel ridicolo vestiro alla cena di gala? Farai ridere anche i polli

Paul thinks he's a business wizard, but he fails two times out of three. Even his latest money-making project is so ridiculous that it's enough to make a cat laugh. My friendly advice: stay out of it!

Paolo pensa di essere un mago degli affari, ma gli va male due volte su tre. Anche il suo ultimo progetto è talmente ridicolo che fa ridere i polli. Il mio consiglio da amico: stanne fuori!

He thinks he's being funny, but he just makes a cat laugh with his lame sense of humour

Pensa di essere spiritoso, ma fa solo ridere i polli con il suo spirito di patata

I'm not going to sing karaoke. No way! I'll be the laughingstock of the pub

Non canto al karaoke. Non se ne parla! Farei solo ridere i polli

In 1893 the British play Charley's aunt opened on Broadway.  The expression “it makes a cat laugh” was created as part of the advertising, to highlight the fact that, although cats do not laugh, the comedy was so funny that even a cat would laugh.