In Chapter 17 of "To Kill a Mockingbird," why is it ironic when Mr. Ewell says he says black people "devalue" his property?

Mr. Ewell is the epitome of "poor white trash," and his family lives in what amounts to a junkyard:

"Maycomb's Ewells lived behind the town garbage dump in what was once a Negro cabin....Its windows were merely open spaces in the walls, which in the summertime were covered with greasy strips of cheesecloth to keep out the varmints that feasted on Maycomb's refuse." 

If he cared so much about the value of his property, he would take some pride in it and clean it up.  The close proximity of the black people in the neighborhood has nothing to do with the poor value of his house and land. Had prejudice not been so rampant at this time and place in history, people could have recognized that the presence of the black neighborhood - better cared for than the Ewell's place - actually improved the property values.