Just that joke, "Are you a mushroom, too?" What's the moral?

This is a joke. There is a mental patient who has nothing to do all day, just wearing a black raincoat and holding a flower umbrella in a damp and dark corner of the yard, just squatting like that, motionless day by day. He didn't struggle to get rid of him, but he was persistent when he had the opportunity to squat back in that outfit with an umbrella. Many psychiatrists and experts came to see it and didn't even answer a word for several days. So everyone gave up and said that the mental patient was hopeless. One day, a psychologist went. He didn't ask anything, just dressed like a patient and squatted with him under a flower umbrella. Every day. So after a week, finally one day, the patient spoke on his own initiative. He quietly approached the psychiatrist and asked in a low voice, "Are you a mushroom, too?"