I don't think Scout ever actually reveals this answer, either in retrospect as an adult or as a child as the story unfolds. We do know that she was a tomboy, and she preferred the company of men over women.
Ladies in bunches always filled me with vague apprehension and a firm desire to be elsewhere... I was more at home in my father's world.
She did do her best to please Aunt Alexandra at the Missionary Circle tea, wearing her pink Sunday dress and a petticoat.
After all, if Aunty could be a lady at a time like this, so could I.
And she displayed her most ladylike manners when she took Boo Radley's hand and escorted him back to his house in the final chapter.
... if Miss Stephanie Crawford was watching from her upstairs window, she could see Arthur Radley escorting me down the sidewalk, as any gentleman would do.
But, with Atticus' guidance, Scout probably did grow out of her love of overalls and become a proper lady.