In a medium-sized bowl, add marshmallow fluff, sweetened condensed milk, key lime juice, and lime zest. Whisk gently until the mixture is smooth and well blended. The marshmallow will give it sweetness, while the lime juice balances everything with tangy brightness. Make sure no lumps remain at this stage.
In a separate clean bowl, whip the cold heavy cream until stiff peaks form. You can use an electric mixer for speed, or whisk by hand for a workout. The cream should be firm enough to hold shape but not overwhipped, which can turn it grainy. This step gives the dip its light and airy texture.
Using a spatula, gently fold the whipped cream into the lime mixture. Do this slowly to keep the fluffiness intact. Mix until no streaks remain, but avoid overmixing, which can deflate the dip. At this point, the dip should look creamy, airy, and well-combined.
Transfer the dip into a serving bowl. Sprinkle extra lime zest on top for a pop of color and flavor. Serve immediately with fruits like strawberries, apple slices, or grapes. Graham crackers or vanilla wafers also make great dippers.
If making ahead, prepare the base mixture and store it in the fridge. For the best results, fold in the whipped cream right before serving to keep the texture light and fluffy. Stored leftovers should be kept in an airtight container and enjoyed within 1–2 days.