No he hasn't. Ignoring for a moment how much I despise the term Mary Sue (which is actually a fan fiction term that's not really consistantly defined), characters like that when used in this context tend to be considered too perfect and to always have everything work out for them and never called out on when they do things that are stupid, wrong, or despicable.
Brian has been shown to be far from being perfect, and in this episode alone he gets called out by Bill Maher himself. Not to mention when he was lectured by Quagmire. Therefore, he does not really fit into that type.
Brian has been shown to be far from being perfect, and in this episode alone he gets called out by Bill Maher himself. Not to mention when he was lectured by Quagmire. Therefore, he does not really fit into that type.