Yes, it is safe to use cosmetics whilst breastfeeding. There is no scientific evidence to show that the use of cosmetics makes breast milk unsafe.
Because cosmetics are applied externally to the skin and hair, rather than swallowed, they can have no detectable effect on breast milk.
Of course, some substances do penetrate the skin or may be ingested (including oral care and lip products) but these are readily broken down and harmlessly removed by our bodies.
By law, the products will have undergone a rigorous safety check to make sure there is no possibility that they can build up to unsafe levels in our bodies. So you can rest assured that the products available to use on you and your baby's skin and hair are safe.