Her usual demeanor may have been colder than the surface of Pluto at night, but she did have a warm side as well; She would endlessly spoil her only daughter like any doting parent would. Unfortunately, she was always a frail and sickly mare, and her deteriorating health eventually led to her death while Dew was still fairly young.
That led to Midnight Dew’s dad increasingly neglecting his daughter as he dedicated himself to his work to cope with the loss.
As a result, she was mostly raised by an ever-changing set of servants with varying levels of care.
All this may or may not have contributed to Midnight Dew becoming the snippy ball of insecurity that she is today.