If the mirror doesn’t have honed edges, buy some Emery Cloth at a hardware store & round off the area where the rope will rub.
The rest of the project is geometry. Think about tying one piece of ribbon around a box, but you want it to go two directions.
Practice on a box and you’ll see you can slide that transition “knot” up or down and sideways.
Now tie two more transition knots and then you have two each direction to slide around. The knots are on the back.
When you’ve slid the knots around where you want them to frame the mirror with rope, then hot melt the knots in place and wood glue any gaps.
I would also consider using fine coated wire, to safety wire the sliding knots in place and to make the loop for hanging it on the back.
Thanks, maybe anybody else with ideas