Shakespeare is trying to show in Much Ado About Nothing that appearances—and words—are deceiving. It takes time and an ability to persevere to get to the bottom of who a person is, as people are most often not what they might seem.
We definitely shouldn't rely on another person's word, as Claudio does with Don John, allowing him to frame events so that it looks like Hero is unchaste. Claudio in this case does realize that appearances can be deceiving, but he himself is deceived in how this is so. He renounces the faithful and pure Hero for using her outward beauty to deceive him:
O Hero, what a Hero hadst thou been,
If half thy outward graces had been placed
About thy thoughts and counsels of thy heart!
But fare thee well, most foul, most fair!
But, in fact, Hero is "most fair" inside and out, a pure, stereotypically 'good' woman. Shakespeare's message here—as in Othello— is that men have to get beyond relying on other men for their opinions of women and actually get to know these...
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