While I don't like Loki as a character, I cannot possibly argue against her inclusion as a Mythic. Hell, if there were only, say, half-a-dozen Mythic heroes total in FEH, Loki would easily be justifiable as one of them. (Askr, Embla, Hel, Thorr, Loki and uh, one of the goats?)
If?
EDIT: Uhh, that wasn't supposed to be a pun.