Ask her to wear clothes that you find sexy.
Attendant to that is learning to suppress any possible jealous thoughts that may arise should she decide that she likes these clothes enough to wear them out on her own.
Mind you, it’s possible that she doesn’t feel sexy in clothes that you find sexy. If she doesn’t feel sexy everything else is moot.
Option 2: (This has worked best for me)
Figure out how to find your girlfriend sexy no matter what she wears.
My wife is not one to wear clothes that many would consider “sexy.” The clothes that she wears, however, are clothes that make her feel sexy. And, wow, she is so much sexier when she feels sexy than in any other circumstances.
If you don’t find your girlfriend sexy unless she wears certain outfits, the issue is a lot deeper than “my girlfriend doesn’t wear sexy clothes.”