In showing us how to live, God provides guidelines for our personal outward appearance: Ye shall not round the corners of your heads, neither shalt thou mar the corners of thy beard. (Le 19:27)
The Hebrew for round the corners seems to mean to shave the sides, and mar the corners seems to mean to disfigure the edges. Initial applications may have been related to hairstyles intended to honor pagan deities, or ways to express deep anguish or grief, as when mourning the deceased. (Le 21:1-5)
Similarly, in the same context God tells us: Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the Lord. (Le 19:28)
The general principle suggests any voluntary self-mutilation or disfigurement of ourselves is a desecration of God’s workmanship (Ps 119:73), disrespectful to Him making us in His image. (Ge 9:6) As children of JEHOVAH (De 14:1), we should treat ourselves and each other with honor and respect (1Pe 2:17a), in both our appearance and conduct.
Further, our outside should reflect our inside: our physical appearance is how we first communicate and reveal ourselves and we shouldn’t send mixed messages; the initial impression we present to others should be consistent with who we are and what we represent. (2Co 5:20) We are the light of the world (Mt 5:14), and our physical appearance should align with this identity. (15-16)
So, to reflect who we are in God, we’re to maintain a clean (Ep 5:3), orderly (2Th 3:7), moderate (Php 4:5), sober (1Th 5:8) outward appearance; we ought not needlessly offend (2Co 6:3), distract (Co 3:17) or align ourselves with any unwholesomenesss or darkness. (Ep 5:11)
Further, we must also carefully avoid applying these principles governing outward appearance in a superficial manner, looking for arbitrary, extra-biblical ways to separate ourselves from the world. In the same way God doesn’t call us to believe differently from the world just for the sake of being different, He doesn’t call us to appear visibly different from the world as an end in itself: this would be divisive, a spirit of variance, unloving and therefore sinful. (Ga 5:20)
To the extent cultural norms are compatible with godliness, conforming helps us relate with others in community and set them at ease, which is consistent with loving our neighbors as ourselves. Yet when we’re tempted to emulate the world in ways which are inconsistent with holiness we should resist. (Ja 4:4)