Is it sinful to perform Hajj on behalf of a mother but not a father?
There is no sin on you for not performing Hajj for your father, because it is not obligatory on you to perform it on his behalf. However, it would be an act of piety and benevolence to perform Hajj for him, as it falls within the general meaning of filial kindness that Allah (Exalted be He) enjoins in the following Ayah (Qur’anic verse):
...and be dutiful and good to parents
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.