I honestly can't tell...

I am neither American nor British. English isn't my first language so I can't tell, but everyone commenting on youtube said her accent was perfect. So maybe you are wrong abput her bad American accent. Just saying

At the beginning of the trailer, I was honestly hearing her natural British accent. As the trailer went on, I realized that she was trying to sound American. I'd have to see the whole movie before I judge, but based on the trailer for this movie and on "The Perks of Being a Wallflower", her American accent is pretty weak. If they really wanted Emma for the role, they could have just let her keep her accent and made the character British.