There were
two things that had a major impact. One was the vaccine. The other one was the massive improvement in care and containment practices between March and May 2020. The case fatality rate dropped substantially; infections of care personnel became much less common. (Masks had a controversial-relevance impact, but seeing as the international community at large didn't get to masking any faster than the US, it's unlikely Clinton would have changed anything here.)
On that second point, Clinton absolutely would have made a difference, because we know that the administration was aware of the pandemic well before the cases started spiking in New York, and we know [1] that Trump autonomously decided not to publicly release that information or tell the CDC to prepare for a major epidemic. That was a major, avoidable mistake. I doubt it would have prevented 100k deaths; rather somewhere in the range of 10k-50k.
Also, Trump really energized vaccine denialists; he was already catering to them in 2016 [2], and a loss in 2016 would likely have significantly reduced his audience on this point. It's not clear how many more people would have gotten vaccinated in a world where Clinton won in 2016, but it might be significant. Densely urbanized blue states led in deaths per capita before the vaccine came out, but now only New York and New Jersey are in the top 10, and my state of Rhode Island dropped from 4th-most DPC to 18th [3] -- I don't recall there being a strong correlation between Republicans and vaccine denialism before Trump, and in fact, the correlation may have reversed since 2011 [4].
1: https://www.nbcnews.com/politics/donald-trump/trump-told-bob...
2: https://www.historyofvaccines.org/trump-and-vaccines
3: https://worldometers.info/coronavirus/country/us
4: https://reason.com/2011/10/04/more-anti-science-democrats-or...