In most jurisdictions, no, you don’t need formal legal representation in opposition proceedings if you’re a citizen or resident with a local address. Foreign-domiciled applicants must typically appoint a local representative even when filing the application in the first place.
Whether it’s required or not, opposition proceedings are complex enough that professional assistance is highly recommended in most cases. The procedural rules, evidence requirements, and deadlines all vary by jurisdiction, and missing any of these steps or submitting inadequate evidence can result in the IP office ruling against you. If the opposition proceeds to a hearing, the quality of the legal arguments submitted directly affects the outcome.