Varun's main weakness in the interview was his lack of relevant and engaging responses. He did not provide any substantial answers to the questions posed, which is critical in assessing his suitability for the role. His communication was also lacking clarity and engagement, which are crucial for a marketing role that involves strategy and client interaction. Varun needs to improve on articulating his experiences and skills relevant to the job description and engage more dynamically in discussions about his professional journey and expertise.