How to decorate a mirror?

Wish you had included a photo because that helps A LOT. Having said that, you didn't include whether or not the mirror has a frame. If it doesn't then... frame it! Install a couple of decorative sconce type pieces on either side. Or, better yet, use some 'floating' shelves on the sides and beneath it and place accent pieces on them. It will create a whole new space.

Here are several projects on the subject. Maybe one will suit you.

https://www.hometalk.com/search/posts?filter=mirror%20upcycle

I saw a mirror on Pottery Barn years ago that was too much for my budget. I bought an inexpensive plain mirror with a wood frame, and spruced it up. I used lace, and string of beads (like the ones you find at Christmas), and glued them on. I finished it off with silver leaf. My copy looked pretty much like the Pottery Barn one at a fraction of the cost.

Yes a pic would have halped. Agree with a frame. Also think about framing it with mitered mirror strips.

There are pretty window clings that you could use (sparingly) to add some color.

If if you like painting get some window paint and some stencils if you can't draw and just put a scene on the mirror

Don't know what you have now, but sometimes just adding a frame can dress up a mirror. Purchase a ready made from Michael's or A.C. Moore using a coupon, and glue it securely to the mirror.

Don't know how you feel about fabric flowers and leaves; but, since most other visuals in that area are sort of square and brown wood (mirror frame, cabinets, piano), maybe something with a more organic shape would lend some balance -- something like silk ivy vine draped over or wrapped around the mirror frame? That would also sort of coordinate with your kitchen border, which is visible through the pass through. If not that, I'd choose a different shaped mirror frame. I love that the mirror reflects your windows!!!

I don’t know if they still sell them but years ago it use to be called crackled mirrors which looked like they had cracks ( black or gold or silver) that you could do a large wall with. My sister who had a very dark living room used these mirrors & panels of wood down the whole one wall. It looked super & gave the room much needed light!

Why not add some small , 3D acanthus leaves made from wood to each corner, then paint them with gold (bright or burnished) paint and add flecks of black in the detailed areas.

You should be able to locate trim pieces such as acanthus leaves from a woodworkers' shop, such as Rockler.com.

Alternatively, can you position a narrow table beneath the mirror with brass or pewter candlesticks, or a low vase of flowers?