There is a misconception that shaving and tweezing cause hair to grow back thicker, but there is no scientific evidence that plucking upper lip hair causes the texture to change. Tweezing removes the hair from the root and it will grow back the same color and texture as the previous hair unless a person experiences hormonal changes.
Hair texture is affected by hormones and genetics. If plucked hair returns thicker or darker, then hormonal or genetic changes within the body caused the changes in the hair, not the act of tweezing. Plucking hair is perfectly safe for small areas, but using tweezers for large areas can lead to ingrown hairs, which are both painful and unsightly.