It would because C is the average and C- means you are below average. Which agency would take a below average candidate if they can help it? If you think C- is unfair for you, you should seek clarification from your boss as to why you are getting the C-. To make it non-confrontational, you can ask it from a positive POV, e.g. ask why you got the C- and how you can improve.

Sometimes, the reason why bosses give C- is because they have no choice. There is a quota of C- to give, and guess what? It's easier to give that C- to someone who's leaving, or switching agencies etc. Not saying that's necessarily the case here, but just in case it is, securing the 'blessing' of your boss can help in your next job. I.e. your new prospective agency will call up your ex-boss, who may defend you.

Hope this helps!