The Mona Lisa was not valued as a masterpiece painting for most of its existence.
In fact, it was not even chosen as a top 100 painting in the museum nor was it considered one of Da Vinci's best works.
It was only when it was stolen that the fanfare caused it to become one of the most recognized and valuable paintings in the world. When it was finally recovered, it was welcomed with its own room and a hundred thousand viewers.
According to this video:
According to this video: