This idea is really off - putting. Fanfiction is a form of creative expression that should enhance the enjoyment of the original work. A 'Harry Petunia Piss Drink Fanfic' does the opposite. It takes something that could be a great opportunity for creative writing and turns it into something that is not only unpleasant but also violates the basic principles of what makes fanfic appealing, such as building on the existing story, characters, and themes in a positive and engaging manner.
I think it's completely unacceptable. Harry Potter is a beloved franchise, and Petunia is an important character in it. To have such a grotesque idea like this associated with them is not only disrespectful to the source material but also to the fans who love the series for its magical and positive elements. It has no place in the world of fanfiction.