[1] A person speaking with a mockney accent might adopt cockney pronunciation but retain standard grammatical forms, whereas the genuine cockney speaker uses non-standard forms (e.g. negative concord).
[2] It is an affectation sometimes adopted for aesthetic or theatrical purposes, and at other times just to sound "cool", to generate street credibility, or to give the false impression that the speaker rose from humble beginnings and became prominent through hard work and some innate talent rather than the education, contacts and other advantages that a privileged background tends to bring.
[2] Mick Jagger is often accused of being the first celebrity in modern times to overplay his regional accent in order to boost his street credibility.
For example, in a courtroom, a more formal voice register with technical legal jargon can be used to intimidate a defendant.
In contrast, mockney seeks to lower the perceived socio-economic class of the speaker.