I am wondering, can I add whipped cream to cream cheese icing , to make it more light.

Or I should just mix the cream cheese and the whipping cream with a little sugar.

I would make the cream cheese icing first, making sure it is well whipped. The whip the cream, do not add sugar, and mix them together.

It would be fairly soft icing, spreadable, and you may be able to pipe some simple borders, but it would be less sweet.

The Whipped Cream (with cream cheese) frosting is very good:

http://www.cakecentral.com/cake_recipe-901-Whipped-Cream-Frosting.html

(I thought 1/2 cup sugar was plenty.)

HTH

Thank you, I will try the recipe JanH suggested.