The earring seems to have various meanings. Kira recognizes Li Nalas' earring instantly in "The Homecoming," which suggests there's some kind of family crest or memorable symbol on them that denotes family or something specific to the wearer. I'm sure I remember Kira saying that Shakaar fashioned her earring for her out of the metal of a Cardassian weapon or something like that, which suggests both that they don't get their earrings right at birth, and that they might symbolize a specific event or achievement or point of maturity. And Quark wore a big extravagant earring for the Gratitude Festival in "Fascination," which suggests that non-followers can wear them for fashion purposes if they choose, at least on special occasions.
And for Bajorans, there probably isn't much of a distinction between "cultural" and "religious" - their religion denies their culture almost entirely, at least for most citizens. So maybe the wearing on the other ear is the only way she could think of to make that distinction.