Both Boo and Mayella are products of their dysfunctional families, restricted to their homes by parents who come and go as they please. Boo's father decided that Boo would be best off inside the Radley house rather than on the inside of a jail cell. Mayella's home must also seem like a prison, since she is stuck taking care of the rest of the Ewell kids while father Bob prowls about the town and drinks up the family paycheck. Mayella rarely leaves her house unless it is to tend her modest garden, while Boo only comes out at night. Boo is accused of "Any stealthy small crimes committed in Maycomb," while Mayella must live with the notoriety of her family name, "the disgrace of Maycomb for three generations." Neither Boo nor Mayella have any friends, and both are innocent of any real wrongdoings until Mayella's loneliness forces her to seek the friendship of Tom, a married Negro man--a relationship that is forbidden in 1930s Alabama. Boo's brother, Nathan, must also believe that Boo's interest in the Finch children is equally unhealthy, but he takes measures to stop Boo's contact by cementing the knothole of the tree. At the end of the story, Boo is forced to kill Bob, ending a chain of events first started by Mayella's own flirtation with Tom.
The first thing that pops into my head is that Boo is a social outsider by choice. He chooses to remain inside and watch the world from behind closed doors and window curtains. He sometimes comes out at night, as in the moment he rescues Jem and Scout from Mr. Ewell. You can bet he stays up on the events of the town with other family members who are out and about each day. He is a good man from a decent family who chooses to sequester himself.
Mayella, on the other hand, is a social outsider for two reasons: one, she belongs to a family that is not respected. In fact, they are looked down upon as the trashiest white family in the county. They live near the dump and close to the black inhabitants of the town. This land would not have much value and no respectful white family would willingly reside there. The second reason is due to Mayella's home situation. She is the eldest daughter, but is treated as a mistress by her father. There has been questionable behavior there which she must know is wrong. Her guilt also makes her an outsider--both the guilt thrust upon her by her relationship with her father, but also the guilt she takes on by lying about Tom in court and causing an innocent man to go to jail.