Sri Guru Gobind Singh Ji said that the stamp of Sikhi is Kesh (unshorn hair). A Sikh is to keep all bodily hair intact and the hair on their head should be covered by a Dastar (Spiritual crown). The Dastar along with the rest of the Sikh dress gives the Sikh a unique identity which allows them to be spotted out from the crowd. The Dastar keeps the hair clean from dust and organizes the hair in a respectful manner.
By Reading the Sikh code of conduct you will see how important Kesh And Dastar are for the Sikhs:
He who as a Sikh places a hat on their head; will be reborn seven times as a leper. – Sri Guru Gobind Singh Ji Thankhaanama Bhai Nand Laal Jee
Those who walk outside eat or distribute food without their head covered will be punished severely. Thankhaanama Bhai Nand Laal Jee
Comb your hair twice a daily and re-tie your turban each time your wear it. Rehatnama Bhai Nand Laal Jee
So these “starch easy put on cap style pug”s are out of the question! A Sikh Normally combs their hair at Amritvela and just before rehras!
The hair moustache and beard are created within God's order. How can those who violate their natural form by cutting their hair ever be accepted by the Lord? Rehatnama Bhai Desa Singh
Those beards are true, which brush the feet of the True Guru... True are the faces and true are the beards, of those who speak the Truth and live the Truth. Ang 1413 Guru Amar Daas Jee Salok SGGSJ
"God has made the human perfect, but the dishonourable destroy it. They will find no place in the Court of God, like the unbeliever, dog and Satanist" (Guru Nanak, Janam Sakhi Bhai Bala)
The Guru's stamp is Kesh and (Khandi di) Pahul, one who removes (their Kesh) is known as an apostate (bemukh).One who shaves the stamp (of the Guru) of their child (i.e. cut their children's hair), realise that person will suffer a terrible death and be known as a ghost. (Bhai Sahib Singh Rahtinama)
"They alone are a Sikh, a friend, a relative and a sibling, who walks in the Way of the Guru's Will" (SGGSJ Ang 601)