I agree with fireheart on this one. Sure, she isn't a likeable character but I also got the feeling from Nishida's monologue that she wasn't cheating (and those screenshots seem to validate this).

She can argue as much as she wants if she is in the right, especially when hanging by a thread in the match and one wrong call could be the end of it. I am sure she played a lot of different A ranked opponents to earn the title of the Queen the first time. There should have been loads of opponents confident enough to call her bluff. If she really was into the habit of cheating, she should have a damaged reputation and zero credibility for contesting calls in the first place.

Yumin's confidence when arguing probably comes from being right most of the time. Sure it is annoying, but it does not make her a cheater.