My wife has let herself go...

Then did you date her out? Bring her to nice places? If no, you expect her to dress up for what? She has time to go shopping? Go for some pampering time like a haircut, manicures etc? You can make subtle hints, like showing her appreciation for all her sacrifices. Gift her a spa session, haircut, etc. Kickstart her back to realizing that she is not just a mum only, and that she is still loved and you still want to date her out, dress nicely and all. You have to realize, some of us mums may have lost confidence after as you said, putting on weight. So don't bother about all these. Plus looking after a kid is busy enough. Where got time to dress up make up and still get kid ready to go out? Somemore go out sure sweat and all, all make up gone. 🤭 Plus we may not be able to fit into the old clothes. So just give up. So just be patient, and try to guide her to finding her old self.