I would get a pattern that you like and trace it onto parchment or waxed paper. Melt chocolate and add a little paraffin wax to it, like you would for dipped chocolates. Pour the melted chocolate into a parchment decorating bag and pipe onto the parchment paper over the design. Chill thoroughly and remove from paper and place on top of the mousse.
I've seen a lot of designs, like butterflies, flowers, etc. made this way and it really dresses up the dessert.