::Well why not? (In fact, the infobox is titled "Johanna Sigurdardottir" FWIW). I don't see any harm in introducing the alternate rendition if it's in general use. Most of us just treat diacritics as their closest English equivalent (é=e, ó=o and so on) anyway. I have no idea what "ð" is, it likes like "o" so I treat it as "o", giving "Siguroardottir". Fortunately the infobox does have a transliteration, "Sigurdardottir", otherwise I'd be blundering around pronouncing the name significantly wrong, and how would that be helpful? (Obviously the "" is just a bunch of squiggles to me (and most everyone) and of no help). But this is getting a bit off topic. I think that most everyone will understand that Frédéric = Frederic, although a few people who don't know English well may not, and maybe for a for a few people I suppose é will render or print as a grey box.