It's not just female characters, it's just more apparent there. Actually I find male characters in games are often sickeningly stereotypical as well. I mean have a look at this:
No matter the gender, protagonists and "good" NPCs will often be stereotypical "good looking". To the point where the villains sometimes look way more interesting than the good guys because the artists didn't have to conform to that unwritten rule.
But games aren't the only medium doing that, you can also find it in a lot of movies... I find it boring.
I believe, however, that a more natural looking character can work just as well, if you have the right audience. Remember the uproar when the first pictures of Geralt from the Witcher 2 were released? Fans were telling the devs that he was way too good looking - they wanted the scarred, imperfect guy from the first game, not some chiseled underwear model. And they listened to their fans and changed the design.
Of course even the changed Geralt is still way above-average in terms of how handsome he is. But it's a little step in the right direction.