Stir together crema Mexicana, mayonnaise, lime juice, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a small bowl; cover and refrigerate until ready to use.

Toss together chicken thighs, cumin, chile powder, 1 tablespoon oil, 2 teaspoons salt, and remaining 3/4 teaspoon pepper in a large bowl until well combined. Let stand at room temperature while griddle preheats. Toss together red onion, poblano chile, 3/4 teaspoon salt, and remaining 1 tablespoon oil in a medium bowl; set aside.

Preheat a large flattop griddle to medium, or heat a large cast-iron griddle on the grill or cooktop over medium-high. Lightly coat griddle with oil. Cook corn, turning occasionally, until tender and charred in spots, 5 to 8 minutes; set aside.

Arrange chicken thighs in a single layer on 1 side of griddle, and spread onion mixture in a single layer on opposite side of griddle. Cook, turning chicken and stirring vegetables occasionally, until chicken is lightly charred, an instant-read thermometer inserted into thickest portion of chicken registers 165°F, 8 to 10 minutes, and vegetables are tender, 6 to 10 minutes. Remove from griddle.

Cut kernels from corn; discard cobs. Roughly chop chicken into bite-size pieces. Toss together rice, cilantro, lime zest, and remaining 1/4 teaspoon salt in a large bowl. Divide rice mixture evenly among 4 shallow bowls; top evenly with chicken, onion mixture, and corn. Drizzle each bowl with 2 tablespoons crema mixture, reserving remaining crema mixture for serving on the side. Top with tomatoes and crumbled Cotija cheese. Garnish with additional chile powder and cilantro; serve with lime wedges.