Preheat oven to 400 degrees. Lightly coat a rimmed baking sheet with cooking spray. On sheet, toss sweet potatoes with oil and season with salt and pepper. Arrange in a single layer and bake until tender and golden brown, about 40 minutes, flipping once.
Meanwhile, in a small saucepan, heat butter over medium, swirling pan occasionally, until golden brown and most of the foam has subsided, about 8 minutes. Immediately pour into a small bowl and let cool slightly. In a medium bowl, whisk together marshmallow creme and sour cream, then stir in brown butter and salt to taste. Serve sweet potato fries warm with sauce and sprinkle with pecans, if desired.
To store, refrigerate sauce in an airtight container, up to 1 day. Bring to room temperature before serving.