A good joke can communicate what a lecture would make unbearable.

Told directly, an unwelcome truth makes people defend. Told as a joke, the same truth gets in before anyone has raised a guard, and they arrive at the point themselves. That is why comedy has always been able to say things nobody else in the room could get away with.

When you need to raise something delicate, consider whether lightness would carry it further than earnestness. The test is whether the truth survives intact. If the joke replaces the point rather than delivering it, you have evaded rather than communicated.