Is Atticus a victim of stereotyping in To Kill a Mockingbird?
I think the answer is a resounding "No". In fact, Atticus seems to be the one outspoken character fighting stereotyping and racism in the community. He is the voice of reason in the novel, and both children turn to their father for guidance and comfort.
Rather than giving into the discrimination rampant in the town, Atticus represents the moral backbone. Already known for his forthright character and commitment to honesty and to right, he refuses to change his attitudes in the face of intolerance. Instead of passing it off to the public defender, he takes on Tom Robinson’s case, determined to give the man a chance at a fair trial. In doing so, he brings his family under the public scrutiny, a scrutiny which directs disapproval on him and on his children. Despite this, Atticus is unwavering in his determination to stand up for his beliefs. He is able not just to oppose injustice, but to see good in the very people who despise him.
The only way I think you could possibly interpret him as a victim of stereotyping would be in the town's attacks on him as a "n-lover" because of his choices. Often, people with intolerant attitudes project their own hatred onto others. The other possibility would be that Jem and Scout see him as an old man, incapable of doing anything "cool". Yet after he shoots the dog, and as they grow aware of the magnitude of Atticus' defense of Tom, they come to realize that their impressions were very wrong.