Is it ok to do Wudu without removing Hijab and without touching the hair?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah enjoins everyone who offers a prayer to be in the state of purity. Allah mentioned the basics of this purity in this verse: {O you who believe! When you intend to offer As-Salât (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to ankles. …}[5:6].
This verse proves that wiping the head with wet hands is an integral part of the ablution and without it the ablution is invalid. Know that wiping the head directly with wet hands is mandatory. In a sound Hadith the Prophet (Sallallahu Alaihi wa Sallam) said: "This is the ablution, Allah does not accept any prayer without it". [al-Dar Qutni and al-Bayhaqi] Imam Ibn Majah also narrated the same Hadith with many chains that strengthen each other. The Hadith proves that wiping the head only once is enough.
If the questioner means is it compulsory to wipe all over her hair, the answer is no. She is not obliged to touch her long and hanging hair. However, she has to wipe the upper part of her head.
Allah knows best.