I liked his emphasis on Johnson as a "trivial" man. I've long felt sure that is the best way to get him.
I disagree with Starmer's other comment because I think Johnson is a "bad" man too. But accentuating that, while making dedicated Johnson-haters happy, will do almost nothing to distress those who voted for him. Some of them even 'like' his badness.
But I don't think they enjoy his triviality. I therefore hope that the Labour party keeps emphasising this. Under the general heading 'Trivial' are all of Johnson's least likeable qualities - his laziness, his lack of empathy, his careerism, his lack of dignity, his privilege and gigantic sense of entitlement, his inability to say sorry, his tendency to go missing when there's a crisis.
It also plays to Starmer's strengths - which are his solemnity and his long career of public service. In that sense he's the opposite of trivial. Those qualities have not been rewarded by the electorate for a long time now, but eventually they will be again. It would be daft for Starmer to try and become something he's not and to compete with Johnson to see who can be the best lovable idiot. He'd lose every time. Better to point out that there's nothing 'lovable' about this clown and to begin the process of convincing the British electorate that they cannot afford to put the destiny of the country into the hands of a trivial man.