What does Scout mean when she tells us that the yard around the Ewell's cabin looks like the playhouse of an insane child ?

Scout is referring to the fact that the Ewell children had no toys. Their yard was filled with garbage.... anything they could find that piqued their imagination..... other people's junk was their treasure.

The varmints had a lean time of it, for the Ewells gave the dump a thorough gleaning every day, and the fruits of their industry (those that were not eaten) made the plot of ground around the cabin look like the playhouse of an insane child: what passed for a fence was bits of tree-limbs, broomsticks and tool shafts, all tipped with rusty hammer-heads, snaggle-toothed rake heads, shovels, axes and grubbing hoes, held on with pieces of barbed wire. Enclosed by this barricade
was a dirty yard containing the remains of a Model-T Ford (on blocks), a discarded dentist’s chair, an ancient icebox, plus lesser items: old shoes, worn-out table radios, picture frames, and fruit jars, under which scrawny orange chickens
pecked hopefully.