Why should you think it's a joke?
If you've done any doorstep canvassing you'll have some understanding of the degree of loathing there is for Corbyn amongst a significant proportion of the electorate.
I had a text conversation with a friend who was canvassing with the Labour candidate in Wyre Forest (a constituency Labour held relatively recently) last night. The slightly jokey conclusion of the canvassing was that, with luck, they'll keep their deposit. The word 'Corbyn' came up in almost every single interaction, with - he reckoned - around a full half of regular/sometimes Labour voters saying that they would not be voting Labour because of Corbyn, and that their vote would be going to the Tories.
If you don't already know that you've not been canvassing, and if you've not been canvassing your opinion doesn't mean shit.