Is complaining to the Prophet, may Allah bless him and grant him peace, about an oppressor considered supplicating to other than Allah?

Mere complaining, without a request or a question, is not supplication (du'a). Complaining to someone after their death is not legislated for anyone, whether the deceased was a prophet or someone else, and whether the complainant is at the grave or far from it. The Companions and the Successors did not do that. In fact, Umar ibn al-Khattab sought rain through al-Abbas, the uncle of the Prophet (peace be upon him), and they did not come to the Prophet's grave to ask him for supplication or complain to him. This act is an innovation for which Allah has sent down no authority.