This is certainly not compulsory and is an individual choice. It always helps if both parties have had some legal advice before the mediation because it means they know the strengths and weaknesses of their case. But, because mediation involves the parties reaching their own settlement, a party without a lawyer should not be at a disadvantage, as they are still in control. They can compromise on whatever terms they like, or walk out of the mediation if they decide to. Nothing can be forced on them.