It is wise of you to ask rather than follow others blindly. I say this because someone who has love for Sikhi would always want his apprehensions answered so as to not hurt Rehat maryada...
As far as the question of whether wearing a cap is forbidden in Sikhi or not, the answer is yes. Our Rehet Maryada forbids us from wearing Hats/Caps...
Also, just to add to this I sometimes come across people who say they can't wear Dastars because they get headaches or they shorn their hair for the same reason. I just ask them one simple question, if you don't like your mother/father, would you replace them for someone you do?
May Waheguru bless us all...
Khalsa ji..Guru Gobind Singh ji gave us gift of turban. Wearing a topi would mean that we did not accept our father's gift. A sikh must always wear a turban. Guru ji said that " rehat pyari mujko, sikh pyara nahi"...