In a 12-inch cast-iron skillet, heat 1 tablespoon oil over medium-high heat. Add corn; cook, rotating occasionally, until charred, about 4 to 5 minutes. Reduce heat to prevent excess browning, if needed. Transfer corn to a large plate; let stand until cool enough to handle, about 10 minutes.
In a large bowl, whisk together lime zest and juice, honey, salt, cumin, garlic, and chili powder. Gradually whisk in remaining ¼ cup oil until well combined.
Cut kernels from corn cobs. Place kernels in bowl with lime mixture. Add mango, bell pepper, onion, cilantro, and jalapeño, stirring until well combined. Let stand at room temperature for 20 minutes.
Place watercress in desired serving bowl. Using a large slotted spoon, transfer mango mixture to bowl with watercress, tossing to combine. Garnish with cilantro, if desired. Serve immediately.