Letting one's self go, as in allowing one's self to get...FAT indicates to others that you simply do not care about yourself, and by extension, care about others.

Now, is it so wrong to not be attracted to the unattractive? And the above just addresses the physical appearance. What about the inner beauty that seems to say, I just don't care how I look or how others see me? I may be stupid and slothful, doesn't that make me a real catch?

Shallowness, superficiality, vain glory, however you wish to describe the selection of others does indicate something of yourself.