What is the irony in the dad coming home with the Cadillac? How does it fit in with the larger theme of the book?

That winter Dad comes home with a gold Cadillac Coupe Deville named Elvis. He claims he won the car in a game of poker. This just illustrates the theme of his incompetence and lack of priorities Rex has. His family is starving and poverty stricken, yet he drives up in a useless Cadillac.