In general it was both. Trump was both a reaction to polarization and helped increase it.

With the pandemic, I don't think there was an polarization beforehand. The US usually rallies around even unpopular leaders during trouble. 9/11 is the obvious example. Who specifically do you think caused the pandemic polarization?

The progressive left saw it as a strategy to attack an opposing party president in an election year. The pandemic was their only real hope of election success. Their candidate field was weak. The economy was strong leading up to 2020. The president was polarizing, but was immensely strong among his base and had cross over appeal to the blue collar middle class and increasing support of other demographics that usually vote Democrat.

I think there is no doubt in my mind that no pandemic Trump would have easily won a second term. You can argue that the pandemic polarization strategy was successful in that Trump lost, but there was a cost. Also considering the significant lack of success that the Biden administration has had managing the pandemic, too, I am of the opinion that the cost of the increased public polarization may not be worth it.