If it's true (I never quite believe all I read in the media) that he was asked a couple of times to put on his mask - did so and then removed it - it's just another illustration of his contempt for the norms and standards of behaviour that we expect from our ministers and politicos. Witness also the photo' of him at the COP26 meeting where he is sitting, maskless, next to other older and therefore more vulnerable attendees. I'm sure he doesn't feel comfortable in a face mask and that's understandable, but others managed it and if he had any respect for them, he could have too.
I don't think Johnson genuinely cares much about anything other than his own personal ambition and believe that he is a ruthless, determined, entitled, individual. He can, and does, do what he wants because he has the support of his majority, many of whom I suspect will forgive or ignore his failings because he has given them what they craved.
I'm not simply 'Boris bashing' - I never like to believe that anyone is wholly, irredeemably a bad person, but I cannot find anything about the man that speaks of compassion, empathy, understanding or consideration for others, or care for the general 'wellbeing' of the nation as a whole. I don't think he's evil or even deliberately nasty, but I do believe he's an unsparing, ego-driven, manipulator, so I am not at all surprised that he didn't obey the rules that applied to everyone else at Hexham.