if the girl lost 50 pounds, then she would "upgrade her mate pool" which would imply that somebody's personal worth as far as relationships go is based on looks.

It's not so much based on the looks as much as what the looks imply, especially if those looks are a result of first world lifestyle choices, like extra body fat content.

Appearance won't tell you everything but it can tell you a lot about a person, especially their lifestyle, health and mentality and above all, how they view or take care of themselves.

For example, I notice a great difference in personality between (formerly) overweight people who are actively losing weight and making effort to get healthy and others. Unless they obsess over it they are generally very motivated and positive and just good people to be around.

On the other hand there's overweight and obese people who know about their problem but are fatalistic and say they're going to die someday anyway or that everything is unhealthy so there's no point. The latter mentality is a drain to be around, no matter how good the person may be otherwise. Then there's the people who are just unaware or pretend to be unaware. I don't have to explain them.

If one is noticeably overweight, you could expect heart problems, low self-worth and drive out of them and even depression or suicide, though whether the extra weight causes the mental condition or the mental condition causes the extra weight is a bit of a chicken-or-egg dilemma.

Now if we assume an established relationship, if the person starts a relationship fit and healthy, but degrades to depressed and overweight, it could be a sign of problems in the relationship or expectations of that relationship with either partner. Usually getting stuck in routine or just taking each other for granted.

Obviously little of the "appearance" stuff matters if you already know and love the person for who they are, but if they are strangers, first impressions make a huge difference. They can also give you an idea of what the future holds for that person.

Lastly, making some effort to take care of yourself shows you love yourself and want to be around to care for the people around you. The cover of the book doesn't matter as much as the contents but it can often tell you what's in the book.