I'm not sure. I know he was a perfectionist with the lines although he flubbed many, he would get annoyed when other cast members screwed up their lines. The appearance of him being anxious was just part of how he wanted to portay Kramer. I don't think he was like that in real life.
One thing is for sure,he was no stand up comedian. After his racist tirade at that comedy club in Los Angeles, his career tanked.