Thanks for reaching out with this beekeeping question! It's normal to find moisture outside of the hive and on top of the outer cover after it rains or snows. At Michigan State University, we haven't noticed big differences between colonies that we winter with a wrap and moisture wicking board versus colonies that we don't wrap or use a moisture wicking system. That said, too much moisture in the hive can be stressful for colonies, and adding insulation on top may have some benefit. 

Personally, if I were to add insulation to the top of the hive, I'd be inclined to place it between the inner and outer cover so bees have space above the frames and so they are possibly less likely to chew/remove the insulation. 

You may be interested in the condensing hive, which is one of several proven wintering systems. It involves adding insulation on the top of the hive. 

Happy beekeeping!