Tile uneven fireplace hearth concrete
Should I try to use mortar and smooth out the surface and make it 90 degree as possible?


NO- any such patch would be likely to adhere unsuccessfully and would crack and come loose.


  ...install tiles with thinset and try to make the tiles as level and as 90 degree as possible by adjusting the thinset amount?


YES- it should be fairly easy to run thick with your thinset, or back-butter the tiles to build it up.
However, I question the need to do so.