What is the ruling on naming a child "Amina" after the mother of the Prophet, peace and blessings be upon him, bearing in mind the impermissibility of naming children after disbelievers?

There is no harm in naming a girl "Amina"; for the default rule concerning names is permissibility, and "Amina" was not exclusive to disbelievers; rather, female companions were named with it. And the fact that it is the name of the Prophet's mother (peace be upon him) does not mean it has a special merit.