What is the ruling on naming a girl after her mother and adding the word "junior" to it, such as "Nawal Al-Sagheera" (Nawal Junior)?

There is no objection to naming a daughter after her mother, as long as the name is desirable or permissible. The basic principle for names is permissibility, unless the Sharia prohibits them, such as names that denote servitude to other than Allah or ugly names. There is no Sharia-based impediment that prevents this. "Al-Sughra" (the Younger) can be added to the name for distinction, as Ali ibn Abi Talib (may Allah be pleased with him) did when he named one of his daughters "Fatima al-Sughra" to distinguish her from Fatima al-Kubra (the Elder).