Covid was a long-running event so it's hard to pick up the effect from the polls, but I think it has to have hurt Trump somewhat. It was a major crisis that he bungled (and the polls showed beyond a narrow margin that people didn't like his response to it) and now something had gone wrong for ordinary people, Trump couldn't just run on a 'great economy'. My guess is that pre-Covid Biden was on track to win the PV by 2-3 points, while losing the election, and if Trump had come across better in his Covid response and worn masks more he would have gotten some of the usual incumbency boost and perhaps lose the popular vote by 0-1 points. Polarisation/stability in views of Trump was so high that his ceiling may have been lower, but it's very likely his ceiling was at least 0.6% higher and so he would have been re-elected if he didn't obviously botch Covid.
The polls in March 2020 had Biden up around 5%. If you take off 3% due to the Trump over performance in polls then you get to a Biden PV win of 2%, but losing the electoral college.
