Wow! Do you ever have a tough assignment! Bob Ewell is actually the antithesis of the mockingbird. Mockingbirds are described as doing just one thing, according to Miss Maudie:
... make music for us to enjoy... (and) sing their hearts out for us."
In fact, he is more like the would-be killer of mockingbirds that Atticus warns Jem about.
"... remember, it's a sin to kill a mockingbird."
Bob attempted to harm or kill Jem and Scout--the youthful human mockingbirds of the story--before the intervention of Boo Radley. Bob much more closely fits the subject of the newspaper editorial of B. B. Underwood, who compared Tom Robinson's death to the "senseless slaughter of songbirds."
With that said, you might be able to argue that Bob inherited his bad ways from his father. (Remember, Atticus once said that the Ewells had been the "disgrace of Maycomb for three generations.") Bob had little chance to improve his own life, or his family's, since he probably lived under similar conditions himself as a boy. In much the same way that Tom was born black, with no chance for an improved life in segregationist Alabama, Bob was born a Ewell--a disgrace to the town. He, also, had little chance to improve his family's conditions due to his inherited family ties. He had few or no friends--even the Cunningham clan had nothing to do with him--and his lack of education prevented him from getting a good job. I suppose you could also argue that several of the crimes of which Bob is accused--such as beating Mayella and lurking around Judge Taylor's house--were only hearsay. This argument is similar to the unproved accusations against Boo, who was thought to kill animals, poison pecans and peep in windows--all without basis in fact.
You have a real chore in proving Bob a mockingbird, but if you are successful, you should definitely consider a future career in debate or law school. Good luck!