If you ever light that fireplace, under the mantle is a horrible idea. 
 

The Arc has specific mounting guidelines. From the PDF:

You can wall mount Arc below your TV or place it in or on a TV stand.

TV stand placement

• Place horizontally on a stable and level surface.
• Keep 4" (102mm) space (minimum) between the top of the soundbar and the bottom of your TV (if placing soundbar below the TV).
• Don't place vertically or on its side.
• Make sure the touch controls are facing up.

Wall mount

You can secure the soundbar to a wall with a compatible wall mount. We recommend the custom-designed Sonos Arc Wall Mount (sold separately). See wall mount.

Mounting upside down is not advised. Were it me, I’d just put it on the mantle. 

I should have clarified, that the fire place is blocked off/unusable. 
 

thanks for recommendation

Glad to hear that. The Arc was sizzling and melting in my mind’s eye. 

Under the mantle will likely ruin your Atmos experience, on the mantle or to the TV.

Maybe this mount?

https://www.sonos.com/en-us/shop/sanus-tv-mount-arc

I assume that the decoration is temporary. I recommend placing ARC on the mantle. Decoration in front of ARC will result in less damage to the audio than mounting under the mantle. Don’t mount the unit upside down. This will reverse L/R and is totally inappropriate for ATMOS.

Thanks. Decorations are temporary. Appreciate the replies. My wife suggested hiding under and I just hadn’t thought of it so wanted to check. 

With respect to speakers, hiding the speaker also hides the best parts of the sound.