Yes it was. It's a masterpiece by one of the greatest artists of all time. There's a Great Art Explained episode [1] on it that's worth a watch.
The Mona Lisa is an unusually clear example. It doesn't even stand out among da Vinci's other paintings. Another example is music. Whether a piece becomes famous, isn't just influenced by whether it sound good. It also has to hit a specific nerve in the current zeitgeist. If the best House track of all time came out tomorrow, nobody would notice, because the genre isn't popular anymore. The artwork has to be at the right place at the right time, or it will be forgotten by history.