In a small saucepan over medium heat, combine the sugar with the lemon juice. Cook until the sugar has dissolved, about 5 minutes. Let cool.
In a medium bowl, combine the cooled sugar mixture with the mascarpone cheese and the lemon zest. Using an electric mixer, whip on medium speed until the mixture is light and fluffy.
To build the trifle, line the bottom of a trifle bowl with a layer pound cake. Brush the pound cake with some creme de cassis liqueur. Spoon a layer of the cheese mixture over the pound cake, and then a layer of berries. Repeat the layers until all the ingredients are used, finishing with a top layer of berries. Chill in the refrigerator until ready to serve. Garnish with chopped mint leaves.