The books are popular but the fanbase isn't nearly as rabid as THG/Twilight/HP. It'll do okay but I doubt it'll make that big of an impact. The fact that the books are AWFUL won't help (the first novel was originally an incestual HP fanfic between Ron and Ginny, author changed names and a few other details and got it published).
More than half a decade ago I used to read fanfics on fanfiction.net and there were some pretty good writers. I can only assume this Clare was from there, and if so, how is it, if you say her books are awful, did she manage to get a deal out of all the other better writers and have that book be an incenstuous HP fanfic?
Something doesn't click?
Also, you wonder why a series like this is picked up, and not some of the other already established fantasy writers/franchises out there. Has anyone read Pendragon? I used to read that and it seems like it could translate very well to screen just the fact it's like a billion books long probably makes it less of a good prospect.