You can either add whipped cream to the icing (it needs to be real whipped cream, the unsweetened kind, not koolwhip) to the icing to help dull down the flavor (it would still be thick enough to ice the cake with and decorate with if you use the right type of whipped cream), or you can buy a brand of icing thats sugar free (they are always significantly less sweeter), i know betty crocker carries them. If your trying to decorate with the icing though, i suggest not looking at the icing by the cake mixes in stores, its not as thick and not made for decorating. You need to look in the crafts section by the cake decorating supplies, they sell buttercream icing there thats made thicker for decorating and holding shape on the cake.
wilton is a good brand to buy from, also betty crocker. you should try to make the icing on your own if you want it specific, its also a fun task
To make icing less sweet and rich, you can add whipped cream to the icing, until you reach the desired texture and taste.
Would the texture still be firm enough to "ice" it? Or make little decorations with it?
Yes, it should be. I've always done it, and so have several other people in my family.