Micaiah is a female character only, which helps to get more aesthetic differences from other characters. She is also a mage in a franchise that would like to have more. Micaiah offers gimmicks of white magic that aren't applicable to other FE Characters. She has a pretty appealing design, I have seen many people that like her design despite don't actually being fans of her character. Micaiah's lack of sword also helps in her favour making her be a more special choice for a character, especially considering FE's reputation as a sword fest. Micaiah is also a popular character, despite having one of the harshest story's to prominence. Being a Mage makes her unique in the kind of attacks she can have, only comparable with Zelda. Her presence would be the end of having to fit the "New FE Character" criteria for each Smash (Smash Ultimate's New FE Character is technically Corrin)