Since I'm not acquainted with this light novel, it's difficult to determine the exact themes. But common themes in such works could be the bond between generations, unrequited love, or the challenges of social interactions. For example, in some similar light novels, the relationship between a parent and a child can play a crucial role in the story, while at the same time, the romantic feelings of the characters are also explored. Without reading 'you like me not my daughter light novel', we can only make these general speculations.
I'm not a reader of novels, but I can answer questions about Korean wives. A Korean daughter-in-law usually referred to a woman who was a daughter-in-law in a Korean family. In traditional Korean culture, the husband and family were the most important, and the wife should play many roles such as housewife, wife, mother, and lover. Korean daughters-in-law usually had to accept strict traditional ethics and values and bear many responsibilities and obligations in the Korean family.
I haven't read this 'you like me not my daughter light novel' yet. Maybe you can check out reviews on websites like Goodreads or Amazon to get an idea of what it's about.
Perhaps she enjoys the creativity and imagination that goes into crafting gay fanfic. It could also be that she feels a sense of connection or empathy with the characters and their stories.