As I said my granddaughter had one but around here we attribute it to Mexican-American descent and it is mostly girls.
Those of African and Asian ancestry can also have the Mongolian spots, so it does not necessarily signify American Indian ancestry. Also, not everyone of other-than-Caucasian descent has them as babies, so it’s really not helpful.
Many Mexican Americans have native American blood, they are metizos, that is, the descendants of mixed couples in Mexico. Most little Mexican babies are born with the Mongolian mark. It fades as they get older.