I think the accusations of sexism only come out when things go wrong. Leah saw her appearance as eye-candy as a chance to save the day, and she very nearly did (by being a better sales person). So why would she then complain about it being sexist? With things looking bleak, Natalie saw it as a chance to fight her way out of a corner by making Kurt look bad (as she had also tried with her "don't be tactical" comment).
Women apply double standards about this (treble standards if you include the sexist comments they make about men). They dress and make up to appear sexually attractive but then complain of being seen as a sex object or eye candy. If they dressed in a more businesslike way it wouldn't happen. If you really want to avoid being treated like a sex object it's probably best not to dress like a sex object and act like a sex object.
No one, female or male, should be on the receiving end of unwanted sexual comments or attention, but if you dress and act in such a way to take advantage then you are in part responsible for how you are perceived.