I guess I’m not that much concerned about the quality rating, except to get better results from filters
Falling rain/snow make some detail in the image harder to see, so it makes sense to drop quality score a bit - if rain/snow can be detected, it already is lowering the clarity enough.
Snow cover is a bit more complicated. On one hand, it does hide detail/features. On the other, you already mentioned regions where snow cover is there for most/all of the year. And in other regions, it can be very useful to see what a place looks like with snow. Unless/until Mapillary decides to publish the detail and reasoning, it’s all just guesses.