Yes, shirk is zulm. The word zulm in Arabic means to keep something in a place other than its original place. Accordingly, zulm could either be related to the rights of Allah or the rights of people.
Although Allah refers to "shirk" as "zulm azeem" (great injustice), it is generally not considered as such since it is directed towards Allah (swt).
Because modernity places man at the centre instead of God, our understanding of zulm is strictly intertwined with man. Upholding the rights of Allah (swt) or addressing zulm related to Him is not considered a cause worth championing.
Those Muslims concerned with fighting oppression should turn their attention towards the greatest oppression: shirk.
Allah says in the Qur'an, quoting Luqman:
"O my son, do not associate partners with Allah. Indeed, association [with him] is great injustice." (Qur'an 31:13)
