Are the jokes inside of Christmas crackers referred to as ‘mottos’? Or are mottos and jokes different? Or is it ‘A joke is a motto, but a motto is not necessarily a joke’?
Merriam-Webster defines motto as:
I have never heard of a joke being referred to as a motto.
However, a quick search indicates this is a UK usage, as is the custom of Christmas crackers.
It appears in this context that “motto” is a general term for the messages in the crackers, which started out as love poems. Jokes are one type of cracker motto.