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.
The term unceremonious marriage refers to two kinds of marriages. It could refer to the marriage which fulfils the pillars and conditions of a correct marriage, but it is not registered in the court (or the mosque). If this is what you meant, then this is a correct marriage, and there is no harm, Allaah willing, to be a witness, so you may honor the request whenever you are asked to witness.
On the other hand, this term could refer to an agreement between a man and a woman to marry without the presence [or consent] of the guardian of the woman, if this is what is meant, then this is not a marriage in principle. Therefore, it is not permissible to be a witness for it, or to accept to be a witness, and whoever does so is sinful and he must repent and regret what he did.
For more details, please refer to Fatwa 81385.