I am 100% sure this is just the translation not coming through. In Asian cultures like Chinese, Korean, Japanese, etc you will see people with kids referred to as "Mother, Father" or "Cindy's Mom, Cindy's Dad". Even your siblings, aunts, uncles will refer to you as such instead of your name.
In Yeni's case, she wouldn't actualy be calling Eddi "Dad" or "Daddy" due to a fetish or kink. It is merely their customs that influenced the writing since it was written for a korean audience.
How far have you read this story?
It should have been explained from chapter 1 that the mc transmigrated into a novel.
In the novel, the Silver haired emperor marries the red haired mage villainous. They have a son and he becomes the "Villain" of the novel world.
The pink hair mc is trying to stop this from happening because she is now the princess of a kingdom destined to be ruined in the future by the villainous son of the emperor.
Hence why the title is "Seduce The Villain's Father". She tries to seduce him to stop him from having a kid with the red haired mage girl and ending the world and her kingdom.
To add on, she refers to him as father because initially, MC is familiar with him as quite literally...a father to a villain from a novel she read. Hence why she would call him "Father" (also note Korean does not translate smoothly over to English and what we are reading is english that has been transformed for easier understanding by the translator). In Korean Culture, they do not refer to people by name directly. You have honorifics. Like -ssi, sunbaenim, noona, hyung, oppa, unnie, "name" mother, daughter in law, etc. To refer to people directly they must be family or super super close. There is also a seniority. These cultural differences will influence how Korean Comics and literature are written and often times, will not translate over to English properly.
Who calls someone they plan/want to marry "father" unless they have some sort of weird fetish ... Like she made up her mind to marry him and proceeds to call him "father"...