Chrysippus was a stoic philosopher known for his good humour. Here's how Wikipedia has his death:
He died during the 143rd Olympiad (208–204 BC) at the age of 73.[4] Diogenes Laërtius gives two different accounts of his death.[18] In the first account, Chrysippus was seized with dizziness having drunk undiluted wine at a feast, and died soon after. In the second account, he was watching a donkey eat some figs and cried out: "Now give the donkey a drink of pure wine to wash down the figs", whereupon he died in a fit of laughter. His nephew Aristocreon erected a statue in his honour in the Kerameikos.[19] Chrysippus was succeeded as head of the Stoic school by his pupil Zeno of Tarsus.
So, I suppose Diogenes Laertius would think either the image, or the joke Chrysippus made, was at least a little funny. Why? Is it because eating figs and drinking is a human activity? Is it because the donkey was already associated with human stupidity (and Chrysippis laughter has anything to do with observing human folly)? Is it related with the undiluted wine? What's the deal with donkey food?
PS: I hope I could add a non-specific question as an extra. Chrysippus was known for being funny, but no works of him have lasted. Are there mentions of Chrysippus' arguments or shenanigans which are humourous?
There is a joke there, though you might not find it very funny.
It's often difficult to explain ancient jokes and there will be different explanations for this one. The basic elements are pretty standard fare for Greek comedy: donkeys are stupid and stubborn, figs are commonly used as a slang term for female genitalia, and unmixed wine - the drink of barbarians - is a great recipe for comedic chaos. The word used for fig in the relevant passage (Diogenes Laertius 7.185) is indeed the word that is often used to mean, essentially, pussy. But that doesn't seem to be the joke here.
Rather, these three elements have something in common. They are all symbols of Dionysos, the god of drunken revelry. Donkeys and figs are associated with him - in the case of figs, the reason should be obvious - and nothing could be more Dionysian than unmixed wine.
Greeks would normally mix their wine with water and honey during their drinking partes; one of the things the guests decided at the beginning of the night was how many parts water to mix with the wine. Drinking wine pure was thought to be the practice of wild barbarians who did not know the Greek virtue of self-control. Some Greeks believed that drinking wine pure would make you go insane. But that, of course, was exactly the point of Dionysos: he was the personification of the uncontrolled party, the wild rave, the mad dances of his female worshippers (the maenads) and the orgies of his lusty mythological companions, the satyrs and nymphs.
In other words, Chrysippos saw two things that reminded him of Dionysos, and saw immediately that a third thing could complete the picture so perfectly that Dionysos himself might just have to pop down from Olympos to give him a round of applause. Donkey + fig: funny. Donkey + fig + unmixed wine: what would be the odds?
If you wanted a modern equivalent, though without the same religious dimension, it might be something like seeing a priest and a rabbi walk into a bar, and asking them if their friend the imam was late.