No I don't think a fbb can be too muscular - however, they can be, and often are too manly. 2 very different things. Personally, I find a big square jaw, over plumped lips, and a deep voice as very unattractive, even if the muscles aren't so big. However, get a cute girl, with a nice girly hip sway, and feminine mannerism, and if she had the most gargantuan biceps it would not be too much.
Real FBB are disappearing because of a social pressure and sport oficials policy. They have to train extremely hard, have less and less opportunities to go on stage, have ridiculously low prizes for winning, and ae repeatedly abused in media. They are punished by male chauvinists, because they do not fit into societal stereotypes - they just do not look as the most men want them to. But it is should not be about wat men wants, it should be about what women wants. They apparently still want to have big muscles, because they despite of everything just keep getting bigger and stronger.
http://www.realfemalebodybuilding.com/real-female-bodybuilding-news/can-female-bodybuilders-get-too-muscular-to-win-the-insane-muscularity-of-patty-corbett-vs-the-perfect-package-of-shannon-courtney.html