There is some flavour text that describes that Fallen Aasimar tend to be evil, but it's not clear whether they must be evil, and conversely, that the other two must be good, and that a Fallen Aasimar cannot be good, etc and where does a neutral Aasimar fit into this?
104-105) have three subraces Protector, Scourge and Fallen. Playable Aasimar (from Volo's Guide to Monsters, pp. This question, in addition to a player of mine who recently wanted to play a Lawful Neutral Fallen Aasimar (but ended up doing something different), got me thinking about something.