Each to their own, I say. Some women feel great and do all sorts of things in the hours/days post partum, some women don't. I certainly wasn't bleeding loads, unable to stand up. And I happily would have had someone come in and do my hair and makeup before I left the hospital. Equally, some people prefer privacy and to stay at home with just their immediate family. Neither is right or wrong.
Maybe Katherine genuinely felt fabulous, maybe she wasn't dripping blood, maybe she wanted to show the world her baby (and who wouldn't want to do that with a face of makeup and styled hair)? Maybe she didn't want to sneak out the back of the hospital. Maybe she simply felt she should do it and picked her battles? Maybe she compromised after discussion with her husband? Maybe she wanted to do more and William dissuaded her? Honestly, no one but them knows.
Plus, Katherine is in a very different position to Meghan, especially as William could well be a reigning monarch at some point and George being 3rd in line to the throne. Meghan and Harry are slightly removed from that, as is their baby.