Porn provides a temporary fix--the visual stimuli given to the user helps that person to have sexual feelings and an eventual orgasm. However, it isn't healthy for overall self-esteem in that porn isolates the individual. Instead of helping someone to socialize and make love in the 3-D world, excessive porn only exacerbates someone's feelings of loneliness.
Although I wouldn't say that pornography is particularly good for viewer self-esteem, I also wouldn't say that it's bad for it. Some people may feel empowered by it, thus boosting their self-esteem. Others may feel as though they need to measure up to the way porn stars look; in this case, it would hinder that person's self-esteem. I think that the latter is more often the case.