Should film editors have a demo reel?
Yes and no.
But mostly no. (Sort of.)
Yes - Editors should have a short demo reel that showcases some of the projects they have worked on in a short, creative and memorable fashion.
It should feature well-known faces, brands, logos and recognisable projects for the "well if they've worked for XYZ, then they must be good" marketing vibes.
It should also represent you in a way that also communicates your personality and artistic approach.