In To Kill a Mockingbird, what does Scout mean at the end that "It'd sort of be like shootin' a mockingbird"?

Arthur "Boo" Radley is a recluse in this story. He avoids people and does not socialize with the residents of the town. However, he is the real hero of the story. He is the guardian and protector of the Finch children.

Scout understands her father's admonishment about shooting mocking birds with the bb guns because mocking birds are not a pest, they just sing pretty songs and are a joy to have around.

Creating a fuss and having the newspaper interview Mr. Arthur Radley about his heroism would have brought an end to Mr. Radley's lifestyle. He would have been out in the public eye, and that would create even more problems for the Radleys.

So, bring Arthur Radley out in the public as a hero would have "outed" him without his consent.  Being a public figure would have "killed" his personality and his lifestyle.