According to Osho, yes—so-called madmen can, and often do, speak the truth. Free of social pretense, they are innocent, sensitive, and fearless, while the 'sane' commonly excel at hypocrisy. History's seers—Jesus, Socrates—were called mad for insisting on truth. So listen beyond labels, help authenticity flower, and note some only act mad; definitions of madness are unreliable.
Yes: people called 'mad' may tell real truth because they don't play pretend, while many 'normal' people hide behind polite lies.