When a person dies, their body is taken to a Mortuary where a Mortician prepares the body to be placed in a coffin for burial.
The body is placed upon a table made of White Marble, which is referred to as a "slab".
It is a thick slab of stone.
Thus, the person referenced by Wodehouse, looked as though they were "dead". Their eyes and face may have looked cold and lifeless, lacking warmth or friendliness.