Rabbi Menachem Azariya miPano in Asara Ma'amaros, Eim Kol Chai, section 3 simanim 9-10 cites an unsourced midrash that Shlomo met Naamah when he was cast out of his kingdom by the demon king Ashmedia (see gittin 68b). The story goes that Ashmedai cast Shlomo's ring, which had the name of God engraved in it, into the ocean. Shlomo the wanderer ended up in the palace of the king of Ammon, where he served as a guard. Naamah chose to marry him but was expelled from the kingdom for doing so (along with her husband). There near the ocean they encountered a fish which had swallowed Shlomo's ring. Naamah finds the ring inside the fish and gives it to Shlomo, restoring him.
This midrash is cited (in abridged version) by Chidah in midbar kadeimot, ma'arechet nun, section 24. The summation being that what was good about Naamah is that she helped Shlomo "get back on his feet" so to speak, though that is never spelled out explicitly. However this aggadah is taken to task by Rabbi Yosef Chaim of Bagdad in his sefer Binayahu on Bava Kamma 38b where he proves (conclusively to my ears) that the story is implausible due to the dates of Shlomo's reign and Rechavam's birth.
Furthermore, to me this "midrash" seems to be a cobbling together of several other well known tropes such as the animal which swallows a treasure which ends up restoring the protagonist or the story of Rebbi Akiva's marriage to the daughter of Kalba Savua.
The gur aryeh to bamidbar 31:2 sees the references to the people themselves, Rus and Naamah, as lav dafka, preferring instead to view the historical themes of the nations of Moav and Ammon leading to the melucha of Dovid and Moshiach