As with most issues in law, it depends. Contempt is available as a remedy if someone is violating a court order. But, there must be a court order in place to violate. So, if your mediation is court ordered then the answer is yes, otherwise, no.
From a practical standpoint, most courts will happily issue an order to mediate by a certain date if either side asks for one. When we feel that mediation could be helpful and the other side will not agree to mediate, we usually file a motion to order mediation and it is almost always granted.