I have heard that it is not permissible to wear a Jinnah cap.

Answer

A Jinnah topi is the headgear of fusssaq. It is not permissible to emulate the fussaaq nor to adopt their dress styles. Mufti Taqi's commission of an act of fisq does not alter the law of the Shariah. When one mingles with fussaaq and fujjaar, one's Imaani inhibition to fisq and fujoor becomes eroded. One then becomes desensitized spiritually paralyzed which renders one incapable of understanding Shar'i realities.

as to the reason/wisdom in following an Imaam?

A. Firstly, since you lack adequate knowledge to respond to and demolish the morons who pose moronic questions, your reaction should be silence. This is the command of the Qur'aan Majeed. Allah Ta'ala says about the Muttaqeen: "When they hear drivel, they turn away from it, and they say: 'For us are our deeds, and for you are your deeds. Salaam on you. We do not follow the jaahileen (morons)." Those who pose such stupid questions are just too stupid for intelligent response.