Many women don't dress as nicely after marriage as they did before. Often practical concerns such as child rearing cause us to dress down. Changing body image as one goes from a young single girl to a mom is also a major concern. But with understanding and a little creativity, you can help your wife define a new sexy style for herself.

Compliment her. One reason she may be dressing down is that she doesn't feel good about her appearance. Be specific about what you love about her body.
If she has a specific complaint about her appearance, don't be glib. Even if it is nothing to you, what she feels is very important. Acknowledge her feelings and encourage her gently to view herself differently.
Buy her a piece of clothing that you would like her to wear. Make sure it is something she will like and make sure it is her size. It must fit her personality too - a mom will not wear something that is made for a teenager.
Take her out someplace nice where other people will be dressed up. Be sure to compliment her. Let her know how proud you are to be seen with such an attractive lady.
You should dress well too. Don't walk around in sweatsuits and act shocked if your wife dresses the same.
Try shopping together online. It can be much less stressful than traditional shopping.

Expecting your wife to dress NICELY is fine. But do not try to push her into wearing something she feels is too revealing.
Many men love high heels but many women find them too painful to wear. If this is the case with your wife, let it go. Don't ask her to break her neck for you.
Pointing out how nicely another woman is dressed is not a good idea.