I should think it's preferable. A house full of obese politicians isn't desirable; it's not a good look for the country from an international perspective, and it probably won't inspire a whole lot of hope in the populace.
While not necessarily true, a lot of people do sub-consciously connect fat-ness with lazyness. We might look at a seriously overweight politician and be inclined to think that they won't work as tirelessly as someone who takes care of themselves. It may be complete rubbish, but there may be a grain of truth to it as well. If someone can't be bothered to make their own health a priority, can we be truly certain that they will make the most of their time in office? Politicians don't have to be Adonis or perfect athletes, but I'm inclined to think that they shouldn't be obese either.
Obviously, a politician's policies, past record and work ethic is the most important thing. That said, if there were 2 identical candidates in front of me - one being reasonable active and in good shape; the other being inactive and fat - I'd choose the physically active person.
As a small example, Ireland's Minister for Health is a rather overweight woman. She used to be morbidly obese, but to be fair she has actually lost a chunk of the weight in the last few months. She is still, however, a far cry from a fit and healthy person. She isn't someone who inspires confidence in the Irish people (for many reasons), and the one thing a lot of people slag her for is her weight. Sometimes, it really is important to be healthy, even if it's just so that there's less political ammunition to be used against you.