In a large mixing bowl, combine the mixed frozen fruit and mini marshmallows.
In a separate bowl, whisk together the whipped topping, sour cream, granulated sugar, and vanilla extract until smooth.
Gently fold the whipped topping mixture into the fruit and marshmallow mixture until well combined.
If using, fold in the chopped nuts for added texture.
Transfer the mixture into a 9x9-inch baking dish or a similar-sized container, spreading it evenly.
Cover and freeze for at least 4 hours or until firm.
Before serving, let the salad sit at room temperature for about 10 minutes to soften slightly for easier scooping.