It's not that populists aren't facing opposition - it's that Farage resigned and said he'd be running in the by-election his resignation caused. He is using the by-election as a political stunt to try and get ahead of an incoming scandal - but by the opposition not running against him, he can't say that his mandate from the electorate is clear, and his only actual opponent is a (well spoken) bin.
Any amount of votes Binface gets is a deep embarrassment to Farage! Binface is just pointing at the farce that Farage has created, and undermining him in a particularly British way.