不是蒙娜丽莎不会见人就笑
[bù shì mĕng nà lì shā bù huì jiàn rén jiù xiào]
A playful reference to the famous 'Mona Lisa' who has a mysterious smile. This nickname humorously indicates that unlike the Mona Lisa, the person doesn't have an easily triggered laughter or smile, emphasizing their solemn and serious personality traits.