That is not very likely. As Lucas, you want to preserve your second jump until you know what the opponent is up to and with his floatiness and the stalling effect of PSI Magnet this is easy. Mirror Lucas is not going to launch Lucas horizontally, which would call for an early second jump. DSmash can be answered with a tether grab without immediate reeling in. When tether grabbing the ledge, mirror Lucas could try to DSmash on reaction once offstage Lucas reels in, but I doubt it's executable, rather it's punishable.
My point is that yes, PSI Magnet gimps are a thing and as a Lucas who's aware of the mirror MU you try your best to prevent it. But dittos aren't necessarily boiling down to who's gimpging first. Lucas has his ways to avoid offstage pressure. It may not carefree, but neither is Lucas doomed to gimp.