Everyone wants the person they're in love with to think they look bangin' hot, and if they truly love you, there's no doubt that they should. Fashion is a category in one's relationship that stands all on it's own. Have you ever dated anyone whose wardrobe was a complete atrocity? That it is thin ice to tread, indeed.
Some of you may argue that what you or she wears is truly not a matter of importance when developing a long and lasting relationship with someone. But, really, in order to keep your attraction alive and burning bright for years to come, it's vital to nudge your love in the right direction if she happens to be a total "closet" case. Just try and not hold her ex against her in the process.
Should you be some kind of weird tyrant dictator girlfriend who lays her outfits out for her? Absolutely not — that's sick and crazy — but if your beloved asks you if she looks good in what she's got on and she's, in fact, violating hardcore with her outfit, you have to tell her. If I asked my partner if she liked what I was wearing and she didn't, and I found out later that she had held back that kind of information from me, I would be devastated.
If something she's got on is straight up unflattering, telling your GF is the correct thing to do. I'm not saying a dislike for your girlfriend's favorite pair of sneakers is reason for alarm, so take it easy and recognize the difference between clashing tastes and clashing on the real. Honesty is always the best policy, but only if it's major. Constantly picking at minor details will cause more harm than good and no one made you the queen of style anyway, so use your good judgment, yeah?