You're changing tenses when you shouldn't. I would suggest keeping the first part of your reply in the Simple past tense: Well, since he goofed around so much instead of studying, I'm not really surprised.

The communication behind the construction is fine. However, pay attention to the tense. Its already been reported that Steven failed. Meaning the cause of his failure must be in the past, and to be more precise in the 'simple past'. 'If he goofed around more than he studied, I am not surprised