It's about a balance, it is also worth noting that looks CAN reflect personality, like how failing to keep basic hygiene basically implies one is unemphatic due not being willing to do even basic care to not smell or dirty up their surroundings to keep other people comfortable or simply reflecting poor living habits (Fat, Coke-nails, yellowed skin due smoking, etc).
Of course there is a difference between someone who looks bad due genetics or having been in accident and getting gravely injured and the greasy ass fat NEET who is ugly by "choice" (admitted or not.), but the fact people judge by looks arose for a reason, just like how stereotypes can arise from reason.
But just like with stereotypes it is ignorant to consider them 100% truths, but people will always gauge first impressions because you have to start somewhere.'
TL;DR: Because the personality reflects what you do, and what you do influences the body, the body reflects your personality. A person very interested in working out and eating healthy will likely do so, and therefore develop a fit body, that kind of person is probably not too interested in a person who hates working out and only enjoys Fast Food and therefore has a obese look.