Yes but only if you approach it the wrong way. Tact is everything. I tell people often to place yourself in your girlfriend’s shoes. How would you like for her to approach you. Use that and make your claim.
There could many reasons she doesn’t dress up anymore. If you want to know why ask her what’s going on with her. Show care and concern. Tell her of ways you have fallen off the wagon and what you plan to do to make it better. That way it seems more or less like its how can we improve ourselves for each other and not what can she do for you alone. She will appreciate it. If it’s weight gain ask her how you can help her achieve her goal.
Can you both take a class together? Could you make a pact to eat healthier and help each other be accountable? Could she just be in a funk and just need some support from you? Absolutely. When you see she tries compliment her on that look. Don’t say things like “finally you’re stepping it up.” Just be there for her.
Also non subtle ways is to purchase her some sexy outfits and take her out. Compliment her on how well she looks and watch her blossom. The goal is to not push her, berate, or belittle her to get what you want. Sit down and have a chat works too. Be careful with word choice though. Don’t say things that put her down. I’m sure she can tell that she’s not her best self too. Also make sure that even if you don’t mention something you once did in the beginning that you mentally take note and make an equal gesture as well.
I had this issue in my marriage and my husband talked to a friend of mine and it worked only because I don’t see an issue with it. We have limits on what we can outsource to our friends. If you are in a relationship that is secure enough to do so then talk to one of her girlfriends. They can suggest an outfit she’s been eyeing but wouldn’t buy for herself. They also can shed light on a place she wants to try but hasn’t. The objective in talking to them isn’t about how to approach her but what you could do to make her happy and feel sexy. I think a girlfriend could appreciate that you spoke to her friends for support and not to bash. Avoid talking to them about how she’s not doing something that she once was. Those are her friends and you don’t want to come off as a jerk boyfriend.
Good luck out there!