For anyone surprised when a used heat shield looks scorched and pitted: that's the point. An ablative shield is designed to char and erode, carrying heat away with the material it loses. A pristine one after reentry would be the alarming photo.

So it's a shield that works by being sacrificed. Like a pan you're supposed to burn.

Pretty much. Which is also why ablative shields aren't reflown. You can inspect one, learn from it and put it in a museum, but you can't send it up again.