What is so funny in this Canto are the author's very ironic and tongue-in-cheek reflections on love and women, which are obviously meant to provoke humour in the audience. This comes from Don Juan's relationship with the beautiful maiden, Haidee, which represents both his and her first love. Thus Byron is able to elaborate on the nature of first love in women, and comment on his observation that actually, although women initially fall in love with just one man, they very quickly move on to loving a number of men. Note how the poet describes this:
One man alone at first her heart can move;
She then prefers him in the plural number,
Not finding that the additions much encumber.
There is something hilarious in the way that "number" and "encumber" are made to rhyme with each other, and the choice of feminine rhymes (rhymes of more than one syllable) is a key way in which Byron achieves humour in this poem. Note too how he goes on to describe marriage:
Marriage from love, like vinegar from wine--
A sad, sour, sober beverage--by time
Is sharpen'd from its high celestial flavour
Down to a very homely household savour.
The comparison of marriage and love to vinegar and wine is one that is hilarious in its slightly blasphemous nature, but it also reflects in an amusing way on how so often marriage can be something that can take the love out of a relationship and distill it down to a "very homely household savour." Humour is therefore present in this Canto through the presentation of love and marriage of the author.