First of all, for feedback of this kind in general, you should absolutely be as truthful as possible, meaning everything you say should be rooted as closely as possible in the truth.
How candid you should be is another matter, and making that decision calls for further assessment of the context. Remember that the purpose of criticism is to be helpful. The more helpful you believe your feedback will ultimately be, the more candid you should feel comfortable being with it. There is little upside, for you or for the others, in unleashing criticism if it will do no good, while at the same time exposing yourself to the downside risks.
Second, while is it possible that it is genuine and will be helpful, superiors soliciting feedback from underlings about peers can be a recipe for disaster. On the surface, it is a potential indicator of a supervisor who is out of touch with his/her staff. A competent and functional one would know already both how the other person is performing, and what the third person thought of them as well. Feedback up and down the chain of command is very different than third-party feedback, which has a bit too much of the air of schoolyard gossip about it.
An incompetent and/or dysfunctional supervisor could create many types of small and great mischief with such a process. He/she could be simply seeking to validate a conclusion already reached about someone. He/she could share your feedback with anyone and everyone, including the subject. He/she could decide that you are actually the problem child based on your feedback.
Bottom line, I would be hesitant to provide such feedback in writing. This stuff can and will come back to bite you in the a**. I might be more comfortable talking about it in a more relaxed and interactive context, such as over lunch or coffee, where you can better read how your information is being received and tailor your message thus.