It is allowed in Islam. According to a Post, which has cited some hadiths as well as given authentication from the verses of Islam. Only Tattoos are Haram in Islam because they are permanent. It is a temporary thing, which fades away after few weeks or days
Mehendi helps to reduce a headache especially if the headache starts in your temple and spreads to the central part of your head. According to Ayurveda, it is known as Pitta headache which is due to an aggravation of Pitta dosha. Mehendi helps to manage Pitta headache by balancing Pitta. This is due to its Sita (cold) potency.