Cersei is not the primary source of Joffrey's cruelty.
The clearest evidence is that, in both books and TV series, her other children Tommen and Myrcella are portrayed as pleasant, intelligent and well-balanced individuals. Both are as "normal" as can be expected given their royal upbringing.
Also, Cersei's "cruelty" is not on the same level as Joffrey's. This becomes particularly clear in the book A Feast For Crows, in which
there are many chapters from Cersei's point of view, as she rules the seven Kingdoms as regent on behalf of Tommen.
Cersei is mean, petty, vindictive, insecure, and paranoid, but she is still mostly a sane and rational human being. Her cruelty is shown to originate from her fear and insecurity. As the audience, we may consider her behaviour evil but we can still understand it.
By contrast, Joffrey is a pure psychopath who inflicts pain simply because he enjoys it. There is no rational explanation for most of what he does.
A good illustration of the difference between Joffrey and Tommen is their attitude towards animals:
In the book A Game of Thrones, we hear the story of how as a child, Joffrey killed a pregnant cat by cutting open her belly. Nobody encouraged him to do this, it was an early indication of Joffrey's own extreme sadism. On the other hand, in A Storm of Swords and later books, Tommen is shown as being particularly fond of cats and he would never harm one as Joffrey did.
The consequences of Joffrey's action are interesting. King Robert hit him so hard that one of his baby teeth was knocked out, but didn't follow up on the matter. Cersei was horrified by the injury to Joffrey and didn't take any interest in the reason why Robert had hit him. Joffrey seems to have learned he could get away with anything he wanted, as long as Robert didn't find out.