The commentor ranking is based on the comments you leave on hubs (not answers or forums or other stuff). A certain percentage of all HubPages users are given commentor ranks. The most important thing is to leave valuable, clear, and useful comments, and not just small phrases or negative stuff. The more you do that, the higher your ranking will grow, over time. HubPages looks at the last 9 months of commenting, so you need to keep commenting steadily to retain and grow your rating.
Hope this helps!