From that scene it appeared to me that the attack was being pinned on Joff, and then he uses a woman for target practice to cement his nastiness in the minds of the viewers.
I wonder if they will use Joff's death to send Cersei spiraling? The show has def made her "softer" by attributing the order to murder Robert's bastards to Joff and pinning the attempt on Tyrion's life on Joff. (As other posters pointed out, Tyrion thought it was Cersei but never knew for sure.)
Perhaps Tywin's quip about her not being as smart as she thinks is the beginning of the build to her walk? She is already suspicious of the Tyrells and has openly regarded Marg with disdain more than once.
I think flashes of her paranoria and lack of forethought are being presented to the viewers to build to C's "mini downfall".