Does rain wash away the THC/resin resulting in weaker product?
THC and the resin produced by cannabis isn’t water soluble.
You can dunk your ripe plants into a lake and the strength will remain the same.
(after drying and picking out all the fish of course)
As @PhillyRock stated. Water has no effect on trichomes. A lot of people dunk their harvest in a water/peroxide mix to wash the buds of impurities after harvest. Your biggest concern with rain on buds is bud rot.