Is not visiting one's parents' graves considered a form of undutifulness to them?

Visiting graves is a prescribed Sunnah (definition) for men, with the intention of taking admonition, remembering the Hereafter, and supplicating for the deceased, without undertaking a special journey for this purpose. Filial piety towards parents continues after their death by supplicating for them, maintaining ties with relatives from their side, fulfilling their bequests, honoring their friends, giving charity, performing Hajj and 'Umrah (definition) on their behalf, and settling their debts. Visiting the grave is not a prerequisite for filial piety; for supplication and other acts reach them from anywhere.