It tries, but... (Books 1-3)
The Mortal Instruments by Cassandra Clare is a series that focuses on a group of humans called Shadowhunters, defenders of humanity from demons thanks to the angel blood they have. Such is the premise of the series, and for the most part, it works. The setting gives way to fantastic storytelling and a great plot. However, there are two problems that reveal Clare's past as a Harry Potter Fanfic writer and works against the series.
The first is that the characters, who are mostly expies of characters from the Draco Trilogy, are supposed to be saviors of the human race, but don't seem to care about them in any way. Simon, a normal human, is mostly ignored by everyone (including his Shadowhunter best friend and female Lead) until he becomes a vampire, and there's tons of Fantastic Racism
towards both humans and Downworlders (magical creatures). The narration never even bothers to call them out, except when it's convenient to create drama between Clary and Love Interest
Jace, or to hypocritically call out the villain, Valentine.
The second problem is the world-building that, while interesting, makes no sense. Shadowhunters are supposed to live among us. They are not Harry Potter characters, and as such have to interact with Muggles (or "Mundanes") and their technology (they sometimes take the subway to move around), yet they have no idea of things like the Internet or TV shows, just because Clare obviously liked introducing the characters to those concepts and seeing them react humorously, even if, as I said, it makes no sense storywise. As a side-note, I also fail to see why the (in-story) world finds Jace attractive beyond shallow appearance, since the narration constantly puts him in a negative light, being quite the Jerkass to everyone but Clary (including, I should note, his adopted siblings).
Bottom line, I think the series has some very interesting concepts, and the story and humor is good, but the two problems mentioned above prevent it from actually being great books. They can be entertaining, however, and some twists and turns were very well thought out. I wouldn't recommend it, but see nothing particularly bad about the story or style (except for some descriptions where the prose turns very purple