Maybe. It depends on her personal taste. If she keeps watching it, there's a good chance she likes it, but she could also be watching out of curiosity.
In a sense, it's not a literal 'watching' as the show is an inanimate creation. But in the context of the show's creators or those associated with it being aware of fanfiction, it can be normal. They might look at it for inspiration or to gauge fan interest.
Regarding Jessica's sister Madison in Supernatural fanfiction, one possibility is that she has a connection to a certain supernatural species. She could be half - angel or half - demon, which would make her a very unique and powerful character. This would add a whole new layer of complexity to the fanfic, as she would have to deal with her own identity while also getting involved in the typical Supernatural chaos.
It could imply that the show 'Supernatural' in some way is associated with or has an influence on the fanfiction related to it. Maybe the show's creators or actors are aware of the fanfiction and are 'watching' in the sense of being aware of what the fans are creating in that realm.
Another theme is the idea of redemption. Some fanfictions have Jessica coming back in some way, and Sam sees it as a chance to make things right, to protect her better this time. It's like he gets a second chance at their relationship, which ties into the whole redemption concept in the Supernatural world.
One common theme is revenge. Since Jessica often meets a tragic end in the original 'Supernatural' series, her sister in fanfictions might seek revenge on the forces that caused her death. For example, in many fanfics, she hunts down demons that she believes are responsible.
One great 'Supernatural' Jessica sister fanfiction is 'Jessica's Hidden Legacy'. It delves into the idea that Jessica had a sister who discovers she has powers related to the supernatural world just like Sam and Dean. The story is full of mystery and adventure as she tries to figure out her place in this new and dangerous world.
Perhaps Jessica returns but she has some sort of new power. For example, she might be able to see the future or has a connection to the other side that helps the Winchester brothers in their hunts. This could be a really interesting twist in the fanfiction.
A great plotline would be if Jessica came back as a sort of supernatural being herself. Maybe she was brought back by a powerful witch, but not fully human. This would create a whole new dynamic as Sam would have to deal with her new state. She could be on a quest to find out who brought her back and why, and in the process, she and Sam could rekindle their relationship in a very different way compared to before.