"He hath a heart as sound as a bell, and his tongue is the clapper; for what he thinks, he utters."
This is a bit of a double entendre, as it seemingly praises Don John for his honesty. However, because Don John is known for being malicious and spreading negativity, the "sound" of his "bell" is actually the harsh and unkind things he says.
This comment is funny because it cleverly mocks Don John's nature, revealing his true character through Beatrice's seemingly complimentary words.