Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper or foil.
Place the potato on the baking sheet, and using a fork, repeatedly pierce the top of the sweet potato to keep it from exploding in the oven.
Bake in the preheated oven until sweet potato is fork-tender and cooked through, 45 to 50 minutes. Set aside to cool.
Cut sweet potato in half once cool enough to handle, scoop out the flesh, and place in a food processor or blender. Add cilantro, lime juice, tahini, garlic, cumin, salt, and red pepper flakes. Blend until well combined and smooth. Taste, and adjust seasoning if necessary.
Spoon dip into a bowl, garnish with chopped pistachios, and serve.