at minimum, increase the width of mantel to that of the TV and put a mantel shelf the entire width. I would increase the hearth to this width as well.
I would increase the height of the mantel sides so that they frame the TV. I would look at the height of the door trim and window trim in the room cap the mantel sides at the higher of two with a piece of trim the same width as the side triim. this is a start.

I would skip the mantle and do a full wall treatment. Something in a darker tone which will downplay the tv. You could do stone, tile, metal etc. If budget is an issue you could do a textured or metallic paint treatment.