This is an extremely subjective question, but, considering how Ewell abuses his children physically (and possibly sexually, given Mayella's statement that her father's kisses "don't count" as a first kiss when coming onto Tom), I don't think he is meant to be all that sympathetic a character.

Ewell is pitiable in his ignorance and hard heart, certainly, but he lashes out by hurting other people and terrorizing those closest to him. He also attempts to murder Jem and Scout, both of them children, just to get vengeance upon Atticus for standing up against him in court. He is a petty, cruel, vindictive man with no positive traits that we, the readers, are told about or shown. He is the closest thing the story has to an individual villain: more a personification of prejudice and ignorance than anything else.