If a woman complains about part of her body or her clothing, she is not looking for you to commiserate with her or tell her that it looks okay. She simply won't believe you.
What she really needs is for you to help her change her state of mind from feeling bad about it, to being happy again.
Look behind what she is saying to understand her rather than only looking at the surface of what she is saying.
She is feeling insecure and lacking confidence in herself at that moment in time. It is a temporary state of mind that could have been brought on by any one of a thousand different events.
You should take this as an opportunity to give her good feelings. The way to do it, is simply to use humor and be playful with her.
An easy way to do this is to "exaggerate" how bad she looks or imply it playfully or so she knows you are making fun of her.
For instance, if she says "My hair looks like hell today", you say "I didn't want to say anything."
You'll usually get a hit on the arm, and she'll tell you you are bad with a big smile on her face.
Then you can go on all night making fun of her hair, talking about how everyone is looking at it, how you're embarrassed to be seen with her because of it, etc...
The point is to show her that it isn't important and to put her in a good state of mind.