Christopher Walken has a pretty distinctive speech pattern himself, but when he chooses too he has proven that he is able to great impressions of other people. He has proven this in his work on Saturday Night Live, as a stand up comedian, and on the many talk shows he has appeared on.
No, I do not agree that Christopher Walken is good at impersonations. He has a very unique sounding voice and unique mannerisms. It is difficult for him to transform that into something other than what it is. Other people have impersonated Christopher Walken very well, but no the other way around.
I have seen many people do wonderful impressions of Christopher Walken. However, the one time I saw him attempt an impression, he just sounded like himself. Either way, he is hilarious in his own right, and doesn't need to add impressions to be an impressive actor. He is talented without them.
No, Christopher Walken is not good at impersonations. There are many people who have done excellent impersonations of Christopher Walken, however. Walken is a distinctive and unique actor who always comes across as himself, bringing his own brand to every role and character. His genius has led other celebrities, including Kevin Pollack, Kevin Spacey, Jay Mohr, Bradley Cooper and others to do stunning impersonations of him.