The DM is the eyes and ears of the players. They're supposed to be able to trust you. This sort of behavior is a betrayal of that trust.
How?
I mean say if they found out?
The DM used method "A" (reskin) to create a new monster, instead of method "B" (create new).
IMO, since the DM had an idea of what he wanted as a foe, a "create new" monster would likely have been very close to the reskin one anyway.