I think a person who doesn't look fit can know how to make people fit even if s/he doesn't bother to do it for themselves. There are also many people with big muscles or flat stomaches who don't know shit, but who are the happy recipients of circumstance and genetics.
OTOH, I agree with you. People are going to operate a bit on an emotional level, so you aren't going to sell them on a diet if you look like crap.
In that regard, I posted some videos I ran across this morning in this thread
from an American news show.
At one point the journalist is talking to a raw foodist guru who looks like a warmed over corpse. The journalist politely excused himself before confronting the guru by telling him that he is only 8 years older than the guru, but that the guru looks elderly in comparison.