It was not serious. I wouldn't write that in a serious fic. Furthermore, if Lucina can have a brand in her eyeball, and the brands can supposeably be anywhere, then why can it NOT be there?
If this is the only argument you have then you're part of the problem. Not all fanfic is "And then they all kissed Kevin and got super pregnant" Very few, actually.
Point I'm trying to make is, like, say somebody wrote a fic that fixed the Sigurd and Deirdre romance development. Would you scoff at it because somebody else wrote an Mpreg fetish fic with horrible grammar? If you scoff at all fanfic because of the actions of a few fanfic writers then that's closed minded. Same goes for scoffing at all fanfic because of a joke headcanon one fanfic writer made. Learn to tell the difference.