Miss Lily Collins.
To say the least, I was rather disappointed to find that Lily Collins – daughter of Phil Collins – would be playing Clary Fray of the Mortal Instruments. As silly as the cover these books seemed to me, I happened to have enjoyed them. As the chief narrator of the stories, Clary is – arguably – the most important part of the casting director’s job. I believe that this was a bad, not to mention hasty decision on their part.
Similarly, I do not believe the right call was made in casting Jamie Campbell Bower as Jace Wayland, but this is not the point. Although much of me wants to say that Rachel Hurd-Wood (and I) would have been a better choice as the lead of the movie, Rachel’s physique does not fit the part. Lily is small enough to play Clary, yet she seems somewhat dirty to me – and I am not referring to her eyebrows. I would have sought an actor a bit more mature as well; I get a distinctly young impression of Lily’s character.
I haven’t chosen to support any particular actress, but Emma Roberts seems to be a far superior choice and I don’t believe she is one of a kind. Of all the many aspiring and successful actors out in the world, surely a better suited actor could have been found. Instead of picking a young woman who colours her hair red from time to time, they could have looked at faces, physiques, charisma and maturity.
I hope that by some miracle she manages to impress, for now I am dubious.

Bellow is a picture I've edited so that her look is more appropriate to play Clary Fray.
Lily Collins, I think I've made her a bit more Clary-ish here.