Oh, if you’re having doubts about the permissability of the mehndi itself, then I would disagree. As long as you’re not doing something that conflicts with the laws of Islam then there shouldn’t be a problem.
I would disagree with the idea that the whole idea of the mehndi comes from the hindus. Even the arabs have a henna party in which the women get together and sing etc. Just because we share aspects of our cultural traditions with the hindus does not make it bad or haraam. As long as we’re not copying their religious ceremonies or doing things that have roots in their religious tradition and it isn’t conflicting with our own religion then there isn’t a problem.