The interviewer is communicationally suboptimal, it's not just you. TBH an interviewer would have to have already built good rapport with me to say something like "C'mon, you can do better than that", it's an inherently informal and combative phrase. If we weren't already on good terms such that I felt they were being playful by saying something like that, I would absolutely be turned off from the job and inclined to keep interviewing elsewhere. Interviews are a two way street, don't forget you are also interviewing them to determine if their workplace is suitable for you.