It sounds like those saying Hepburn was miscast are really saying the character was incorrectly written, as Hepburn is indeed the perfect choice for that version of Holly. If Monroe had been cast instead, for a Holly more like the one in the novella, no doubt she'd be perfect for that version of Holly. Or at least that's my attempt at mediating the issue. ☺
No, I don't think Audrey was perfect for the movie version of the character. I never read the book, so I can't compare the movie Holly to the text one. All I know is that I don't think she fits the movie character well. She's supposed to be a Texan, for God's sake! Yeah, I know they try to explain away her lack of accent,. but I still don't feel Audrey works in the role for a variety of reasons.
Monroe could've pulled off the character's mix of flightiness, sexuality and world-weariness much, much better, methinks.
And dammit, I actually like "My Fair Lady"! I guess I'm the only person who likes Audrey in "MFL" and thinks she's weak in "BAT"!