Does ultrarunning require a rail-thin emaciated body type?
Not at all! If you look at the entire field in a typical ultra, you'll find runners representing every imaginable body type. Most of the elites, both male and female, tend to be on the thinner side. As soon as you get to the next-fastest group, all bets are off. All BMIs are represented.
Losing weight generally makes you more efficient, thus faster, but ultras are run at a slow enough speed to partially negate this effect. I'm not exactly thin myself. The last time I checked, by body fat percentage was somewhere around 15%. My 'ideal” body weight is around 155 pounds. Currently I weigh around 185-190 pounds. In fact, I've gotten quite a few 'if HE can run an ultra, I certainly can!” comments over the years. What can I say? I like food. And beer.
Don't let your body weight dissuade you from running an ultra!