How do I make a swag to cover up big crack on mantle mirror?

You could apply an applique to cover it. I would either do something in both corners or go from the corner down the side to look more like a design choice instead of a patch job. You can get some static cling film that looks like stained glass or mosaic. It applies with water and pinches off with no residue for removal.

For the holidays, you could use a pine bough swag and drape it so the crack is covered.

Use a small nail to gently tap into the corners of the top of the mirror. I say gently because once glass starts to fracture it will continue to keep going in the direction of the crack. Use fishing line or carpet thread to add large wooden beads, attach faux greenery, or anything you fancy. Tie the ends to the two nails on the top to cover the crack.

You could place a command hook on the frame oneither side of the upper part of the mirror and use a soft panel of chiffon, netting, or material that coordinates with your room decor to swag over the mirror crack. This could be changed out depending on the season and flowers, etc. could be added to it as you wish.

Hello. I wonder if some sort of wreath or garland might be considered.

Hello Jax, how about some faux vines, you can make them look as though they are over growing the mirror, or you can try this repair method, simply drape them down from above the mirror

here is info Jax

https://www.youtube.com/watch?v=clNwsELi4GM

How about imitation leading for stained glass, here it is at Amazon, they probably have it at craft stores:

https://www.amazon.com/faux-stained-glass-supplies/s?k=faux+stained+glass+supplies

Amazon sells a self adhesive metal strip that you could form to cover the crack but without a picture I don't know how well that would work. I would be concerned that the structual stability of the mirror could be compromised (how safe is it).

I would make a green garland and then you could add things to it throughout the year (like little ornament bulbs during Christmas, leaves during the fall, etc.)

You could try with Applique using paper or Fabric or glass or all three. Have fun.

Get your self some branches paint them white tie a pretty bow around them add some greenery and what ever you like and you have a sway you could also make one in ell shaped or round

You could add a wooden corner applique. Wood U Bend makes a ton of them that you just heat and glue them on. You can stain them or paint them to match the frame. Just an idea.

Some evergreen boughs would look really nice and you could decorate now with a Christmas theme. In Spring, change up the decor to reflect the new season.