Somebody else suggested *Always write feedback as though it will be read by the person it is about.*  This is excellent advice. It's your path through this rather strange situation.

When "reviewing" your manager, write three specific, positive, and true things that relate to your own accomplishments. "Steve reviewed a draft of my XYZ project plan; his useful suggestions kept me from going in the wrong direction."  That kind of thing.

And be done with it. If your manager deserves a bad review, this is not the place for it, and you are not the person to do it.

You've already spoken to your manager's manager about the situation. She is not going to say, "OK, you're right, let's just stop doing this." You've expressed your misgivings and given her something to think about. That's good.

Now you should just finish the task. Don't worry about being wrong or right; you aren't in a position, yet, to reform the way your company handles these reviews.