According to Osho, the serious face on the mala is a compassionate device: to make you truly laugh, he must remain outwardly grave so you don’t dismiss laughter as a joke. He turns laughter into sadhana, asking you to approach the divine smiling, while inside he is nothing but dance and laughter. The seriousness is method, not his essence.
The photo looks serious because he keeps a straight face so you’ll take laughing as a real practice and actually do it—inside, he’s all joy.