You can even possibly argue that Morecambe’s ice cream van isn’t a genuine one liner as the siren is the set up. Without it – no joke.
The only thing that defines a ‘one liner’ is that its short and funny (just like your mum). But its typically a short funny response to a situation so they can be a funny response to something. Its not all self contained within the one line.
Tim Vine’s jokes for instance – he sets up a scenario and follows it with a one liner. His set ups are also very succinct but the one-liner is still the second bit.
Can’t remember where I first saw or read this but I thought it was pretty good . In response to his partner saying that he never gives her compliments she is told ” For a fat bird you don’t sweat much . ”