I don't think it is accurate to define it as 'mob rule.' Generally, in mob rule, the mob decides who is guilty and punishes them arbitrarily. In jury nullification, the mob decides who is technically guilty but unjustly punished (because the law is bad, because the punishment is too severed, etc..)

I won't lie to a judge or to anyone in court. But I won't ever convict someone if the death penalty is on the line. I expect to be excused, but if not... That isn't 'mob rule.' That's a power granted to me as a juror by the system. Presidents and Congress have grasped far more power than that without explicitly (and sometimes without even implicitly) being granted that power.