All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is not an obligation on a woman to cover her head while performing ablution except if she is in the presence of marriageable men. Please refer to Fataawa: 83702 and 83033 about the Hijab in front of marriageable men.
It is known that wiping over the head is one of the conditions of ablution without which the ablution is invalid, so it is an obligation to wipe over all the head as the Prophet did so. In case the woman is wearing her head cover (Khimaar) and it is wrapped around her neck, and it is difficult for her to take it off, then it is permissible for her to wipe over the head covering instead of wiping over her hair. It is reported that Um Salamah wiped over her head covering while performing ablution. This is the view of the Hanbali school; however, the majority of scholars are of the opinion that it is not permissible for a woman to wipe over the head covering while performing ablution.
Allaah knows best.