Heat the oven to 425°F. Toss the sweet potatoes with the oil, chili powder, cumin, and a big pinch of salt and roast until tender and browned at the edges, 25 to 30 minutes.
Stir the beans into the hot sweet potatoes and return to the oven for 5 minutes.
Whisk the sour cream with the lime zest, lime juice, and salt.
Fill the tortillas with the sweet potatoes and beans, drizzle with crema, and top with cilantro and pickled onions.
Kids like theirs with shredded cheese instead of crema.