Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is forbidden for a Muslim woman to display her adornment before non-Mahram men since Allah The Almighty Says {What means}: "And tell the believing women to lower their gaze and be modest, and to display of their adornment only that which is apparent, and to draw their veils over their bosoms, and not to reveal their adornment save to their own husbands or fathers or husbands’ fathers, or their sons or their husbands’ sons, or their brothers or their brothers’ sons or sisters’ sons, or their women, or their slaves, or male attendants who lack vigour, or children who know naught of women’s nakedness. And let them not stamp their feet so as to reveal what they hide of their adornment. And turn unto Allah together, O believers, in order that ye may succeed." {An-Nur, 31}.
Usually, Hijab fashion shows are attended by non-Mahram men, accompanied by music, televised and filmed and all of these are prohibited since a Muslim woman would never accept to be looked at lustfully by those whose hearts are diseased. Allah The Almighty Said {What means}:" O Consorts of the Prophet! Ye are not like any of the (other) women: if ye do fear (God), be not too complacent of speech, lest one in whose heart is a disease should be moved with desire: but speak ye a speech (that is) just. And stay quietly in your houses, and make not a dazzling display, like that of the former Times of Ignorance; and establish regular Prayer, and give regular Charity; and obey God and His Apostle. And God only wishes to remove all abomination from you, ye members of the Family, and to make you pure and spotless." {Al-Ahzaab, 32-33}.
However, if this fashion show involved no violations of Sharia, was attended by women alone and the shown clothing adhered to the Islamic dress code, then it is permissible. And Allah, The Almighty Knows Best.