This and resemblance to the character, within reason. I wouldn't want a non Chinese actress playing Mulan. But she doesn't have to look exactly like Mulan.

I kinda dislike the tendency to dreamcast actors who played similar characters in popular franshichise. Such Emilia Clarke casted as Kida because of the wig she wears as Daenerys. Or Sophie Turner as Ariel.

This has nothing to do with the movie, it's just related to the actress' reputation imo.

Doesn´t matter at all. It could be a new actor/actress that has an amazing acting talent and physical resemblance.