It absolutely was a bad trade and stupid decision.
Defenses had NO other receiver to respect than Lamb. None. They traded away a quality starting receiver and replaced him with one legged Gallup, a 3rd Rd rookie who apparently doesn't know what the line of scrimmage is, and the likes of Dennis Houston and Simi what's his face.
Not only did they get peanuts for Cooper, but they did NOTHING with the pick they acquired and the cap space they cleared, did nothing to replace him as a starter, AND even deigned to resign the other WRs Dak had a rapport with like Wilson and Turner.