The quote you mention pertains to Aunt Alexandra's view of Scout. To Aunt Alexandra, Scout was born "good," meaning that she was born without all the supposed bad habits she has recently picked up. One bad habit of Scout's that Aunt Alexandra frowns upon is Scout's tendency to go about in breeches or overalls.
Aunt Alexandra is especially upset that Atticus doesn't share her concerns about proper feminine attire for little girls. For her part, Scout complains that Aunt Alexandra only wants her to play with "small stoves" and "tea sets," while wearing the "Add-A-Pearl necklace" she received from her aunt when she was born.
In Aunt Alexandra''s mind, however, a baby is perfect because she's a blank slate, and that baby maintains her goodness (as she grows) by conforming to certain expectations. As Scout is bereft of a mother, Aunt Alexandra deems it her duty to teach Scout the ways of a dignified, Southern young lady. In return, she expects Scout to heed her teachings. Among other things, Aunt Alexandra expects Scout to be a "ray of sunshine" in her father's "lonely life."
Aunt Alexandra's definition of true womanhood is narrow and strictly autocratic, meaning that she never takes into account Scout's desires, Atticus' thoughts on the subject, or Scout's upbringing. To Aunt Alexandra, a woman can only be a "sunbeam" if she's attired properly in a dress, meaning that she can only be seen as behaving appropriately if she's clothed in an accepted manner. Because Scout has continually failed to conform to this expectation of Aunt Alexandra's, she is considered to be a disappointment, hence Aunt Alexandra's pronouncement that Scout was "born good but had grown progressively worse every year."