Are Ibadi/Ibadiyyah Kharijite?

Yes, according to the sects classification done by

• Al-Shahrastani in his book al-milal wa al-nihal
• Abd al-Qahir Ibn Tahir al-Baghdadi in his book Muslim Schisms and Sects

Ibadi/Ibaddiyah belief about Imam Ali (may peace be upon him) agrees with the core Kharijite principle

Ibadis agree with Sunnis in approving of Abu Bakr and Umar ibn al-Khattab, whom they regard as the two rightly-guided Caliphs. They regard Uthman ibn Affan as having introduced bid'ah "innovations" into Islam, and approve of the revolt which overthrew him. They also approve of the first part of Ali's caliphate, and, like Shi'as, disapprove of Aisha's rebellion against him and also disapprove of Muawiya's revolt. However, they regard Ali's acceptance of arbitration at the Battle of Siffin against Muawiya's rebels as un-Islamic and as rendering him unfit for the Imamate, and they condemn Ali for killing the Muslims of an-Nahr in the Battle of Nahrawan.