I think it’s only natural that the general population thinks that their leaders are doing well at a time of crisis. They look to them for guidance and for some optimism that there will be an end to it all in the near future.
Therefore, I don’t find it surprising that, at this point in time, both Johnson and Trump are viewed as generally doing well - in many ways, it’s been the most prime ministerial/presidential that either of them has been. Even it Italy, apparently the public thinks that Conte is handling the crisis well!
The day of reckoning will come afterwards, in the cold light of day when people can reflect on what went wrong and who was to blame. I think we will find that there were plenty of missed opportunities, particularly in the early responses to the virus.
Then I think you’ll see their popularity ratings tumble.