Drizzle the corn with olive oil, salt, and pepper. Place it on the hot grill, along with the poblano pepper. Grill the vegetables, rotating occasionally, until there are grill marks on all sides of the corn, and the poblano is blackened and soft. Remove the vegetables from the grill.
Slice the kernels off the corn. Remove the stem and seeds from the pepper, and dice the flesh.
Toss the corn and chopped pepper with scallions, tomatoes, a squeeze of lime, and more salt and pepper.
Assemble the tacos with the corn-poblano filling topped with cotija and cilantro. Serve with lime, avocado, and your favorite salsa.