Almost everyone who is a parent has said something along the lines of “and if Alice put her hand in the fire / jumped off a cliff would you copy that too?”.
I disagree with this, and I think you're giving Cummings one hell of an easy pass here. A child copying another child is in no way the same situation as adults looking at the man who was de facto running the country and seeing someone who'd decided that the rules didn't apply and didn't matter.
The politicians and I know Cummings isn't one of those, but bear with me ask us, the public, for vote for them on an understanding of expertise and competence at running a country. This includes handling things if something unexpected happens, like a pandemic. There is hugely significant additional expectations on the behaviour of a politician, and the people that surround him or her, than there is over an eight-year-old and their friends.
The Cummings incident, and Johnson's refusal to sack him at the time, did more to break public trust and goodwill than anything else over the whole year. It crosses political boundaries; Conservative voters as well as Labour, leavers and remainers, etc. It was massively, unfixably destructive. (For balance, I also agree that Ferrier getting on that train and Corbyn going to a dinner party weren't smart either. But neither of them were in government.)
Remember after Johnson was ill and came out of hospital? There was a considerable amount of collective goodwill behind him at that point, even from people who wouldn't normally support him or vote Conservative. The Cummings story smashed all that to bits.
Jake Jones's metaphor of the colander is a good one although he does have form for writing from an angle of 'the general public are stupid' sometimes, but then so would I if I was trying to flog books and columns but I think he's missed the gnat trying to swallow the camel here, by and large. And if this situation is a dam turning into a colander, it was Cummings who blew a massive great hole in the structural wall of the thing in the first place.