Arrange the frozen berries in a single layer on a large cookie sheet. Allow to sit at room temperature for 20 to 30 minutes, or until completely thawed.

While the berries are thawing, place the cake pieces in a large bowl. Set aside.

Place the cream cheese in a medium-sized bowl. Add the powdered sugar, and, using an electric mixer, beat at low to medium-low speed until well blended and all the lumps are gone.

Fold all but 1 cup of the whipped topping into the cream cheese mixture. Chill the remaining cup of topping for use as a garnish.

Pour the cream cheese mixture over the cake pieces, making sure that all of the pieces are well covered. Set aside.

Transfer the thawed berries to a medium-sized bowl. Sprinkle the sugar over them, and gently stir until well mixed.

Remove a third of the berries--about 1-1/2 cups--and add to the cake mixture. Using a large spoon, gently mix the cake, cream cheese mixture, and berries together.

Spoon a third of the cake and berry mixture into the bottom of a 3-quart glass serving bowl. Gently spread a third of the remaining berries over the top of the cake mixture. Repeat the cake and berry layers twice, so that you have 3 layers of cake mixture divided by 3 layers of berries.

Cover the bowl with plastic wrap, and chill for at least 2 hours. To serve, arrange the reserved whipped topping in dollops over the top of the trifle. Garnish with the sliced strawberries and mint leaves, and serve immediately.