Preheat the oven to 450 degrees F. Put the poblanos on a baking sheet and toss with 1 tablespoon vegetable oil, 1/2 teaspoon cumin and 1/2 teaspoon salt. Roast until they begin to brown and soften, about 20 minutes.
Meanwhile, put the corn on another baking sheet, brush with the remaining 1 tablespoon vegetable oil and sprinkle with 1/2 teaspoon salt and the remaining 1/2 teaspoon cumin. Roast until slightly golden, 15 minutes; turn, add the scallions and roast 15 more minutes. Let cool.
Cut a 2-inch slit lengthwise into each pepper, remove the seeds and open the peppers slightly. (Peel the skin, if desired.)
Remove the corn kernels from the cob; add to a bowl along with the scallions, then scrape in the milk from the cobs using the back of a knife. Stir in the crema, cilantro, and the lime zest and juice. Season with salt.
Spoon the corn mixture into the peppers. Sprinkle the filling with cotija cheese. Return the peppers to the oven and bake until the cheese is slightly golden, about 8 minutes.