Preheat oven to 400°F (200°C). Toss sweet potatoes with olive oil, cumin, smoked paprika, and salt. Spread evenly on a baking sheet.
Roast for 25 minutes, flipping halfway through, until tender and slightly caramelized.
Warm black beans in a skillet over medium heat with a splash of lime juice and salt for extra flavor.
Stir together sour cream and lime juice to make the lime crema.
Warm the corn tortillas in a dry pan or over an open flame until lightly charred.
Assemble the tacos by filling each tortilla with roasted sweet potatoes, black beans, and shredded cabbage.
Drizzle with lime crema, add avocado slices, and garnish with fresh cilantro before serving.