I imagine what was regarded as beautiful in 1503 is quite different than now. That's why she looks "plain" to many people today. Persistence of perceived historical value determines what is "better" or not, and what has more value than other creations over time. 500 years from now most of the artwork today will probably be considered pretty drab, but some of the art that achieves fame will still be considered masterpieces from our time, and therefore, not drab.
I imagine what was regarded as beautiful in 1503 is quite different than now. That's why she looks "plain" to many people today. Persistence of perceived historical value determines what is "better" or not, and what has more value than other creations over time. 500 years from now most of the artwork today will probably be considered pretty drab, but some of the art that achieves fame will still be considered masterpieces from our time, and therefore, not drab.