Some characters might be amused. For example, Sans would probably make some witty remarks about the fanfiction's plot twists. Papyrus, on the other hand, might be a bit confused at first but then get really excited if he finds a heroic role for himself in the fanfiction.
In a comical fanfiction, most of the characters would enjoy it to some extent. Sans and Papyrus would be great additions to a comical story, and they would probably laugh along with the funny parts. Frisk, depending on how they are portrayed in the fanfiction, would either be the source of the comedy or be reacting to the absurd situations. In a more serious and political - themed fanfiction about the Underground's society, Asgore and Toriel would have deep thoughts. They might discuss among themselves how the fanfiction's view of their leadership and the society they built compares to their own intentions.
It adds variety. The undertale fandom is huge and diverse, and frans lemon fanfiction offers a different take on the characters and their relationships. It gives fans who are interested in this particular pairing more content to enjoy.