The way Lee describes Ewell, it is hard to imagine that she wants us liking him. At best, she wants us to be amused with his antics, and at worst, to hate the guy. She isn't very flattering in describing any of the Ewells, and when Bob gets up on the stand, the descriptions aren't any better:
"a little bantam cock of a man rose and strutted to the stand...his face was red...his nose was thin, pointed, and shiny; he had no chin to speak of-it seemed to be a part of his crepey neck...he reminded me of a deaf-mute."
This description, coupled with the antics he pulls on the stand, making abrasive and shocking comments to rile the crowd up, being defensive during Atticus's cross-examination, and not understanding half of the words spoken to him, make for a not-so-pleasant impression. Later, when he curses at Atticus at the post office, spitting right in his face, we dislike him even more. Then, of course, is the traumatic ending and Bob Ewell's role in it all. We also hear, through Mayella's testimony, about how he beats her, is a drunkard, and never takes care of his children. So, if Lee wanted us to think kindly on Mr. Robert Ewell, she included all of the wrong details! I hope that helps.
Lee is trying to sway her readers not only to dislike Bob Ewell, but also to understand how he came to be so violent and close-minded. His poverty is not a proud one, like that of the Cunninghams, but rather, we know that Bob Ewell is hiding his true shame through his own brash actions and words. He takes pride in mouthing off in an ignorant and backward fashion, and issues idle threats to those who disagree with him.
In contrast, Lee has portrayed the Cunninghams in such a way that their poverty is only a secondary consideration. They work hard, grow and harvest their own food, and survive off the fat of the land. They pay their debts using crops or labor compensation, as illustrated by the nuts that Walter Cunningham gives Atticus. Lee generates sympathy for the Cunninghams, and intense dislike for Bob Ewell, despite their economic similarities.