Preheat the grill to medium-high heat, then brush the poblano peppers and corn with the olive oil.
Place the poblano peppers and corn on the grill, then close the grill and cook for 5 minutes. Flip the corn and peppers and cook for an additional 5 minutes with the grill lid closed, until the skin of the poblano pepper has blistered and the corn has grill marks.
Cut the corn from the cob, then clean the poblano peppers. To clean the peppers, peel the skin from the peppers once they are cool enough to touch, then split the peppers open and remove the seeds. Once clean, dice the peppers.
Whisk together the mayo, sour cream, lime juice, chili powder, and salt in a large bowl, then add the corn, poblano peppers, cilantro, onion, and cotija cheese. Toss to combine, then garnish with additional cotija cheese and serve.