Also different cancers require different chemotherapy with different side effects which again vary according to each person. Even a year after finishing chemo, one active day floors me for two. Kate may well be spending her time asleep on the sofa and that isn't worse, just different.
I'd also point out just how much of a microscope royal women are under for their physical appearance. If Charles appears out of sorts it's a discussion, but more of a "the King isn't looking well" and some hushed tones about funeral plans.
Just look at how obsessive people got over not being given explicit details of Kate's medical history, paparazzi shots, a released image, or even a bloody video of the woman. We've seen claims of body doubles, AI, photoshopping a different face on, plastic surgery, a wig, etc. If she's seen with thinner hair, weight gain, weight loss, swelling, or any of the varied physical changes possible it'll be a discussion point on Twitter, in magazines, all the like.
For the sake of her self esteem and just not having to read random people on Twitter claiming to be experts and that she's dying and being replaced with a hologram I'd not be surprised if she's just not going to be seen until she's photo op ready.