It is natural for a woman to look her best. I think you hate when she looks good because you feel threatened by other men finding your girlfriend attractive. If she let herself go, didn't dress up, and so on, you would probably feel less uncomfortable, but that would simply be detouring your problem rather than solving it. You need to find out why her being attractive to other men threatens you and makes you sad.
I have the sense that your unconscious mind expects you to be dropped by her, and the sadness that you feel whenever men look at her is the emotional reaction that comes up when your unconscious mind imagines losing her. You aren't conscious of the fear, you only feel the sadness. The sadness you feel is tied to loss, loss you suffered when you were young (your unconscious mind is recalling that loss without your knowing it), and loss you are expecting to suffer in the near future (when she leaves you).
Next you need to find out why you expect to be dumped. Did one or both of your parents abandon you emotionally or physically? If so, you would be primed to expect another abandonment; after all, if your own parents didn't stick by you, how can you expect a girlfriend to do any better? Perhaps your parents abandoned each other. Maybe they separated and/or divorced when you were young; maybe they cheated on each other (this could explain your terror over another man taking her away); or maybe they left each other emotionally, if not physically. This experience would program your psyche to believe that relationships can't last, it's only a matter of time before they end.
When you understand the origin of your feelings of fear and sadness and you identify the exact nature of the abandonment trauma that you suffered, you are well on the path to resolving the problem with your girl. Beware of the danger of making your worst fear come true. If you keep trying to control your girlfriend's manner of dress, she is going to resent you, which will increase the risk that she will leave you.
I have given you the key to set yourself free. Soon you will be able to separate your past from the present. You will be able to enjoy her beauty and feel pride in knowing that other men are drawn to a woman who has chosen you and only you.