Q. Is there a source that the indented part of the top lip that people have today is a result of the malach hitting us on the lip in order that we forget the Torah we were taught in the womb by the malach?

A. The Talmud (Nida 30b) mentions only that when the child is about to be born, a malach comes and strikes him over his mouth. It does not record anything about the indented area or philtrum that the vast majority of humans male or female, have over the upper lip and under the nose. Although it is a widespread folkloric notion, it seems unlikely to be right, since as mentioned on a prior answer; the teaching of the Torah by the angel is part of the mitzvah of learning Torah, that applies only to Jewish males. (The indented area is also present in fetuses before their birth)
As far as we can ascertain this lore does not have any basis in Talmudic or Midrashic sources. However, Maharam M’Pano in Ma’amar Yonas Elem (P. 99) mentions from Cabalistic sources an elucidation that would seem to uphold that the indentation is indeed created by the malach. (Contributed by R’  I. J. Diena)

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a