Don't worry too much about it, as long as you are generally eating healthy it's fine. The only thing to keep in mind is that accutane can cause spikes in your bad cholesterol and triglycerides naturally, and eating junk food alot might exacerbate the issue. If your cholesterol and triglycerides go too high, your doctor may take you off the medication (although most doctors would expect a gradual increase during your course, it goes back to baseline very quickly after your stop your course.