"Mass gatherings" refers to multiple events, where each event is attended by large numbers - "masses" - of people. If you just want to have smaller gatherings, you would prefer
We should reduce the size of gatherings, for COVID-19
note the use of a comma - it helps make it clear that the gathering isn't "for" COVID-19, as silly as that may sound. Probably not necessary. Maybe if you're speaking to Trump fans.
As Mari-Lou is commenting, you could look at "have fewer gatherings" also - or, if you want both,
We should have fewer, and smaller, gatherings, due to COVID-19.
We should reduce the number of gatherings we are having, and keep them small, because of the risk of COVID-19.
Countless variations will work.