TV Mounted above fireplace mantel - neck strain?

As a decorating motif it's a cliche.  As a practical matter I know people who say it works fine.

Depends on your situation. If you're watching leaning back so your head is at a comfortable angle then probably fine. You just want to avoid looking up, as anything the least bit uncomfortable for a short time can annoy over time. Unless you get used to it. So I would put something there, sit and pretend and see how it feels. 

Neck strain or eye strain (having to look ’up’ from straight ahead) will be a function of screen center height, viewing distance, and viewing angle(any upward angling of the eye).

I don’t have my big screen over the fire place, but I do have it wall mounted because of room constraints.

I did some calculations for my room and got the screen center set to give zero eye angle up with my comfy couch. No neck strain or eye strain.

Depends. If you are close and the screen center is high up - they yeah - it could be an issue.

I won't get into WAF or other decorating preferences.  Strictly from a 'human factors' standpoint - it can be done.

All of mine are at eye level except office.
There, top shelf = neck strain.