Scout Finch qualifies as a "mockingbird" in the novel because she does not act out of malice or selfishness. She is, essentially, harmless and in need of protection. These are the qualities that define the figure/symbol of the mockingbird in the text.
Along with Atticus Finch and Mr. Underwood, Miss Maudie discusses mockingbirds:
"they don't do one thing but sing their hearts out for us."
Atticus tells his children that it is a sin to kill a mockingbird, implying through this admonishment that these creatures are in need of protection (unable to protect themselves).
We see this need of defense in the novel's strongest examples of "mockingbirds", Tom Robinson and Boo Radley. These are characters that do no harm to others, yet suffer from negative reputations and accusations levelled at them from the townspeople.
Robinson and Radley are not equipped to defend or protect themselves, for various reasons. Scout, as we see in the novel's climax, shares this quality.
When Bob Ewell attacks Jem and Scout, the children need protection. It is Boo Radley who comes to their aid, ensuring that these two harmless children make it home alive.
Boo Radley...turns out to be their protector.
Just as Boo Radley and Tom Robinson are defined by their good deeds and their aid of others, Scout comes to fit this same role as she walks Boo Radley home at the end of the novel.
Very much like the other "mockingbirds" in the novel, Scout seeks to do good and needs more protection that she can give herself.