First of all, I think the OP is confusing "OC" with "Mary Sue" (or Gary/Marty Stu, for the male equivalent). A lot of OCs are Mary Sues, but OCs aren't Mary Sues by default. Make the distinction, please. Otherwise, non-fanfic fiction (hint: all of the characters in regular fiction are OCs) as a whole would be littered with horrendous characterization, and we'd all be screwed.

Now, what I really don't understand is why the hate is always directed at the OCs in fanfics but never the idols' characters. In my experience, when an author writes a poorly-characterized OC, his/her characterizations of the idols are only marginally better, if not as bad, or even worse, especially in the case of being "too perfect." Or is it just accepted in fandom that our oppars and unnirs are literally flawless, so they it's fine and dandy to characterize them as unparalleled paradigms of human perfection, but any OC who comes along who is too many brownie points (or gold stars) above Literally Average (average height, weight, IQ, grades, household income, number of friends for the local/relevant demographic she belongs to blah blah blah) is a ty character (because heaven forbid that anyone who is not an idol be outstanding in any way)? Ah, the joys of double standards.

No, seriously. Take any fic with a Mary Sue in it. Okay, so...

Kim Hana is pretty, rich, intelligent, shy, and innocent as a newborn lamb.

Time to bust out the rotten tomatoes? According to the cantankerous (sorry, I love that word) readers, yes. Now, let's read on.

Kim Myungsoo is a devilishly handsome kingka who is at the top of the school in academics, a piano prodigy,  and the star of the basketball team (can I say l o l because...lol), and all of the girls in the school love him.

Wait, what's this? Not a rotten tomato in sight? And it appears a sizable number of the tomato-tossers have decided to grudgingly abide by the OC and read the fic because L, the Epitome of Masculine Beauty, is in it.

Does anyone else see the problem with this picture?

The OP of that confession complained that the OCs always get the guy/girl, are always pretty, are always special in some way, and are always good at everything (or nothing). Now here in the same exact story as the OC being complained about, we have a kingka who is all of those same things to an even greater extent (he's not just good at everything, he's the best/a prodigy/the star, and he has his pick of all of the girls, not just one), but there are no flames for him? This reeks of misogyny, if anything.

The fact of the matter is, for every Mary Sue OC within a fic, there is probably a Gary Stu male lead, if not twelve (EXO, anyone?), and in fact, considering these endemic kingka/queenka overpopulation problems on AFF, overall, the idol Mary Sues/Gary Stus almost without a doubt outnumber the OC Mary Sues (and rare OC Gary Stus) on AFF, yet no one, not anyone, seems to ever lodge a complaint against them for being obnoxiously perfect. News flash: Even when a fanfic does not contain an OC at all, it is perfectly possible for one or more of the characters to be Mary Sues/Gary Stus or simply flat-out poorly characterized without being obnoxiously perfect or a blatant self-insert.

Thus, I believe the rotten tomatoes should be thrown at poorly-developed characters, whether male or female, idol or OC, instead of OCs as a whole. (Hint: if you hate all OCs without taking the time to figure out whether or not they are actually poorly characterized, you're being prejudiced. Now there's a thought to chew on.) Or, following a more optimistic and helpful attitude, we should direct those authors to some good writing advice about characterization so that we can reduce the damage that the invasive species of Mary Sues and Gary Stus have caused instead of wasting our figurative tomatoes and causing an unpleasant mess...Haters gonna hate (I usually caution against using this phrase, but in this case, it really is prejudicial hatred), tomaters (contraction of "tomato-throwers" that I invented on the spot, so sue me) gonna tomate what is english.