Who rates the Commo Sergeant?

Usually the XO will rate the Commo Sergeant, as they have the closest relation and grasp on your duties. PSG's, generally, do not have that connection as they usually hold formation and then send you on your way to perform your duties and daily tasks. The next in line as your Senior Rater should be the Bn Commo NCOIC, and the Reviewer will be the SIGO. Rating schemes "should" follow a basic guideline that all of the Companies in the Battalion follow.
However, many different Companies do their business differently from this and it cannot be counted on as the final word. There may be a reason that your Company rates in this manner, and it could be to your advantage. Your XO, though, "should" know your job functions better than anybody in the Company. He/She is often the one in charge of maintenance operations, so their knowledge of what you do on a day-to-day basis should be quite vast.
You should address your chain of command for further in-depth reasoning as to how the rating scheme is the way it is for you. Personally it would raise questions that I would want answers to for me as well.

The next highest in you CoC. PSG is appropriate as you are graded on Leadership not just job

Find the SSG or SFC next up in your technical chain of command. I would latch on to him or her for these types of questions. I am assuming that you have a technical chain of command as well?