Til, or beauty spot, on Hip (Kamar or Nitnb) is interpreted as bad in Hindu culture. Luck or unlucky factor depends on the position of the beauty spot on the body. Also the position of the beauty spot have no real significance in many Hindu cultures.
It is widely believed that if the til or beauty spot is on the hip then the person will not have a long life.
Are such things mentioned in Hindu scriptures? No. These are traditions followed by some communities in some regions. Such things are not followed by all Hindus. There is also no scientific backing to such ideas.
Should I believe in them? No you don’t need to believe in such things as it has nothing to do with the core teaching of Hinduism.