Put corn in a 5- to 6-quart wide pot, then add water to cover and bring to a boil, covered. Remove from heat and let stand, covered, 5 minutes. Transfer corn with tongs to a cutting board. When cool enough to handle, cut kernels off cobs with a large heavy knife.
Meanwhile, whisk together lemon zest and juice, butter, honey, salt, and pepper in a large bowl until combined.
Wash quinoa in 3 changes of cold water, draining in a large sieve each time. Cook quinoa in a 4- to 5-quart pot of boiling salted water, uncovered, until almost tender, about 10 minutes. Drain in sieve, then set sieve over same pot with 1 inch of simmering water. Cover quinoa with a folded kitchen towel, then cover sieve with a lid and steam until quinoa is tender and fluffy, about 5 minutes.
Add quinoa to dressing and toss until dressing is absorbed, then stir in corn, scallions, mint, and salt and pepper to taste.