The term (Mary Sue) has been twisted to encompass a number of "dislikeable" or "unrelatable" character traits. When it comes to characters like Fiona (just as an example, since she has been used in this discussion), she can fall under those two traits for some people.
I personally am unsure what catch-all term would best describe these traits in fictional characters, but I do use the descriptor of "Soap Opera Syndrome", if applicable. Fiona for example, has had so many unique events and occurrences befall her, that she
can seem to some, as unrelatable. Similar to many Soap Opera personalities who end up blind, kidnapped, found a lost twin, came back from the dead, kidnapped again, child was switched at birth, husband/wife/lover leaves/gets murdered/gets kidnapped/cheats/cheats again/etc....you get the point. In a soap opera, where that sort of thing is a main staple, it is fantastical and farcical for a reason. And people somewhat expect it. In books, movies, series, or story based games, there is a fine line as to how far many who consume these fictions, will "allow" those soap opera elements to go. If that line is crossed (even if it is just perceived to be crossed by only a few), those characters might end up vilified. At least to a degree.
As others have pointed out, toss in the general disdain that some have for elves and mages, and Fiona is painted with a giant bullseye on her back. My own personal thoughts are, she isn't exactly my favorite character (and mostly based on that which I have gleaned from others, of her portrayal in
Assunder), but I certainly don't have it out for the poor lady. I also don't feel I can fairly judge her anyways, since I never read the book, and her part in DAI just isn't meaty enough for me to elaborate on.
Just my .02. Nothing more.