We know for a fact(well based on this report from a Chinese univesity[1])
that there are two strains of covid-19, I wonder if the less aggressive strain has been around awhile more than the more aggressive strain, it may have been around a lot longer than the outbreak was announced (think fall 2019) and may have been misidentified as the flu/bad cold. And it would explain why some people are getting extremely mild cases, they either have the less agressive strain or already had it but now have the antibodies to fight it. This is total conjecture mind you and the weather aspect is also interesting, look at Hawaii's cases and death rate(580 cases to 10 deaths, they are both very low numbers).
That study is really not very good and doesn’t tell you anything.
Sure, mutations happen all the time, however since the virus has error correction those are rare and very unlikely to have any effect on the phenotype.
[1] https://www.forbes.com/sites/lisettevoytko/2020/03/04/discov...