How hot is too hot above fireplace
i have never seen a zero clearance unit without a door assembly already attached.  was this unit open hearth design and then you retrofitted it with the door/glass?  or was the door/glass just missing and you replaced it?
is it a gas unit or wood burning?  the gas units can get pretty toasty right at the top above the trim escutcheon, but it could also be you don't have a tight seal on the new door to the old firebox, thus letting some combustion gas out and giving you too much heat, plus a little carbon monoxide for good measure.  if its a wood unit, you don't need to worry too much other than the heat degrading the tile mortar or adhesive.  either way, it should never get above 45 deg c, as that's the limit for temps on combustibles (tile no, but wood behind tile, yes) around a zero clearance unit