Most Helpful Girl
Well you can tell her, is just a matter of telling her with the right words. You see most people don’t take it nice when other just criticize (usually they take it personal/hurt their feelings). So instead give a constructive opinion and suggestion. So if you will tell her, make sure you have good reasons to support your opinion and have really good and attractive suggestions for her. For example, you can tell her: “you know you’re very beautiful but I think your dresses hide your beauty. I think a dress like this/like that will match your personality and your beauty a little bit more and makes you look more classy...” and so on...
So let me ask you dear, what is wrong with her dresses according to you? Why are they ugly?
Most Helpful Guy
If you mean they don’t quite fit her right, suggest she get them tailored. If you mean you just don’t like the style it’s better to wait till she wears something you like & heap on the praise. Or ask her to wear a style you like but don’t act like she needs to do it all the time.
Some dresses only look right if the proper under garments are worn for that dress too so that could be her issue. But better to ask innocent questions than to try and force anything.
Personally I can talk about this kind of stuff with SO without issues because she is 1,000% sure that I love her & like how she looks