I'd be inclined to agree with you, but any head injury can be serious. Just look at what happened to Natasha Richardson. Mrs. White might not have the upper body strength like Colonel Mustard does but, if she managed to swing the candlestick and catch Mr. Black on the temple, it could have resulted in unconsciousness or even death. The pterion is the weakest part of the skull and if she was mad enough, it might've only taken her one good blow.

[ john may not have realised they've started playing sherlock's version of the game. it might take a few more turns until the penny drops, but this is more fun. ]

Plus, Mrs. White's card makes her look very, erm, stocky. More than the other women anyway.