Yes. because MockSerializationResult uses MockHttpServletResponse internal methods that don't come from the inferface (HttpServletResponse), like that:

public String serializedResult() throws Exception {
		
		if("application/xml".equals(response.getContentType())){
			return response.getContentAsString();
		}
		
		if("application/json".equals(response.getContentType())){
			return response.getContentAsString();
		}
		
		return null;
	}