Are lockdowns justified?

[FlatAssembler]

His constant lying about the seriousness and the extent of the pandemic

He had no way of knowing. Based on the previous experience, when "experts" warn about a dangerous pandemic, they turn out to overestimate it beyond any proportions. It is only this time that they got the order of magnitude right.

his refusal to wear a mask

Based on the information he had, such as Fauci saying "Masks provide some protection, but they do not provide the protection people think they provide. And given that when wearing masks people tend to be reckless with them and touch their faces more, masks may even be counter-productive.", that would have probably been the right thing to do.
In fact, from the research I have done, I would say it probably still is the right thing to do. The recommendation to wear a mask does not seem to be based on rigorous science. The most rigorous study about that done to this day, the Danish study, found no effect. It seems to be based on unjustified assumptions about both hard sciences and soft sciences. Cotton masks work only if COVID-19 cannot survive outside of a droplet, which has not been proven (although it is probably true). And they also work only if people who are forced to wear masks care to keep them clean, which is a huge assumption.

his disdain for science and scientists

Well, you must admit that "experts" cited by the mainstream media have a horrible track record when talking about pandemics. And I do not think untested scientific theories should have more weight than the principles of good governance. I also think most people know about computers just enough to realize their potential, but not enough to realize what the limitations of computer modelling are, and are putting way too much confidence in it.

his repeated assurance that the disease would miraculously disappear

When did he claim that? As far as I know, it is the people who advocated lockdowns who claimed that, that the disease will soon disappear once we have a vaccine.

and his stated desire to underreport cases and deaths for political reasons

As far as I know, it was Andrew Cuomo doing that, not Donald Trump.