Preheat grill to medium-high (about 450°F). Brush corn with mayonnaise; sprinkle with 1 teaspoon of the salt and 1/4 teaspoon of the pepper. Grill corn, covered, turning occasionally, until charred and tender, 10 to 12 minutes. (Kernels may pop.)

Brush onion and jalapeños with oil; sprinkle with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Grill onion and jalapeños, covered, turning occasionally, until charred and tender, about 5 minutes.

Cut kernels from corn into a large bowl. Chop onion and jalapeños; add to corn kernels. Stir in cilantro and lime juice; serve immediately.