In a large mixing bowl, beat the softened cream cheese with an electric mixer on medium speed until smooth and creamy, about 1-2 minutes. Gradually add the powdered sugar and vanilla extract, continuing to beat until fully incorporated and fluffy.
Gently fold the thawed whipped topping into the cream cheese mixture using a spatula, until evenly combined and no white streaks remain. Be careful not to overmix to maintain a light, airy texture.
Add the prepared strawberries, blueberries, and raspberries to the cream cheese mixture. If using mini marshmallows, add them now. Gently fold everything together until the berries are evenly coated, being careful not to crush the raspberries.
Transfer the mixture to a clear glass serving bowl. Cover and refrigerate for at least 30 minutes to allow flavors to meld (optional but recommended). Just before serving, sprinkle with graham cracker crumbs if desired.