Options to cover imperfection on metal door around new lock
metal door != plaster

Plaster (or I suspect, drywall patching compound) is for drywall. Would work somewhat on wood. Metal? Not so much.

I wouldn't use a white cover - it won't be exactly the same white, unless you paint it too. I'd recommend a metal cover in the same (or similar) finish as the lock. Something like this plate goes around the front back and side of the door:



or something like this circle to cover up just the area around the lock: