May couldn't have turned down the battle against Ash.
She had her Dad's honor to think about. What wouldn't the mob think about her, and her dad, if she turned down a battle? Besides, May was so worked up over all that ego boosting-ness from the mob, that she just wanted to prove herself. And the more the crowd cheered, the more she got worked up. Pretty much similar to when Ash gets overconfident.
And Ash couldn't exactly turn down the battle either. He had his own honor to think about. He battled, and won, against numerous of trainers, and if he turned down a battle against May, then he would've been labeled as a frightened trainer, who doesn't dare to battle against the daughter of the Petalburg city's gym leader.
The fact that the crowd prefered to lie to themselves, and only make things happen to their likings, is just not something you can do about.
May didn't control the crowd. May had nothing to do with their obsessions. And May couldn't help that she, a young naive girl, ended up believing what the crowd said. She isn't exactly the pessimistic type. Not in these kind of occasions anyway.
But if you have to blame anyone for this infair battle, then blame the mob.