I don't mean for the Product Owner to attend the Retrospective, but to also provide his feedback items during Retrospecitves, yes or no?
More than that. He or she would discuss shared feedback with other team members, and together they would collaboratively seek improvement. A good Scrum Event is not a channel through which opinion is squirted at others.
I feel its mandatory for a Product Owner to attend the Scrum Retrospective as an ordinary member.
He could help with inputs when Definition of Done is being discussed.
He could provide valuable feedback which would help the team identifying and implementing improvements.
Moreover since he is responsible for optimizing the value of work done by the Development Team it would certainly be valuable for him to attend.