I am not sure if it was always a criticism of him as much as an accurate reflection on much of his work. Perhaps there was a romantic ideal in his time regarding tragedy and melancholy... now we diagnose and treat that perspective.
Jude The Obscure, is there a more accurate telling of a life unlived, merely through accident of birth? This is a sad story to which Hardy brought his powers of observation and depth of understanding.
A brilliant author and poet, I love Hardy, but he really was an old miserablist, even his comic novel, The Trumpet Major, had to have a downbeat ending