Til, or beauty spot, on eye is interpreted as either good or bad in Hindu culture. Luck or unlucky factor depends on the position of the beauty spot on the right eye or the left eye.
It is widely believed that if the til or beauty spot is on the right eye then the person will have good relation with the opposite sex.
If the til is on the left eye then person will have bad relation with the opposite sex.
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.
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.