Since when was it 'evil' and 'malevolent' to point out reality?
Catherine is not reading mumsnet in her jammies on a Sunday morning. She's not going to be crying into her coffee that some anonymous women on Mumsnet thinks she looks frail and thin. This discussion is not going to affect her in the slightest, so let's stop all the ridiculous hand-wringing.
The people who are affected by this sort of sycophancy and denial of reality are plenty of real women and young girls who do spend their lives on the internet and are fed photos of a real life princess (and in our patriarchal world, becoming a princess by marrying your Prince Charming is still, depressingly, a life goal for many) who is demonstrably considerably underweight and yet praised for her figure and style. They look at these images and want to emulate them.
By not being honest about the fact that Catherine is underweight, by praising her for being so, and using words like 'fit' and 'healthy' to describe someone who is medically underweight, we perpetuate the narrative that being unhealthily thin is desirable, and we encourage vulnerable women and girls to attempt to achieve the same appearance, triggering eating disorders.
None of this is Catherine's fault and I'm not saying that it is - but the reality remains that she is underweight, it is obvious, and I don't agree that we should all be expected to not mention it in the cause of female solidarity, because there are many other women who are harmed in the process of pretending that her body shape is healthy.