Why is it so amusingly terrible?
To start with the casting is hilarious. Everyone in Shadowhunters is young - even the characters who are supposed to be old. Madame Dorothea, the psychic who is supposed to be very old, has been transformed into Dot, Clary's moms' cool friend who lives in their building and is said to be 'like a sister” to Clary.
Even more amusing, Hodge, who in the book is described as “old and haggard,” is played by an actor who is 31!
Also, it seems there cannot be any characters who are too young either, both Maureen and Raphael who are supposed to be younger teens in the books are both 18+, because all characters must be dreamy and of legal age.
For example, in the third episode a character gets kidnapped by vampires and the Shadowhunters go rescue him. In the book, this is a huge fight scene with a whole clan of vampires, a pack of werewolves and the Shadowhunters. Then the kidnapped character escapes because he turns into a mouse! But in the TV show, they just beat up a couple of guards and then go out onto the roof into the sunlight, because vampires don't do sunlight, that's it. BORING!
Or you could read the books instead.