"She can't just BUY CLOTHES and WEAR THEM, they need to be specially TAILORED, which takes MONTHS!"
In the photo of contention, she's wearing jeans, a sweater, and a jacket/vest dealio. Nothing looks specially tailored for her, she's just wearing clothes.
If she needs to hide an ostomy bag, you know what would be great to wear? A sweater, with a jacket over it.
It's not 1960 where a photo of the princess wearing normie clothes is going to break people.
Not to pile on when other people have made the point, but have another look at what she's wearing and the context

She's wearing a very fancy sparkly dress and a tiara, but there's a couple of other elements on show - the big blue sash and the yellow pad with the brooch on it. They're not just random outfit choices.

They're to show Catherine is a Dame Grand of the Royal Victorian Order and a member of the Royal Family Order of Elizabeth II (both were bestowed by the Queen). They were both honours bestowed on her by the late Queen, and these sorts of swags and swashes are to make up for the fact that most Royal women don't serve in the military and so don't have ceremonial medals to wear - because this is a state event and she's in full ceremonial garb. That's because this was the Diplomatic Reception. Same reason she's wearing a tiara, which is generally considered to be one of the most formal items a woman can wear (and most of the royal jewels are historic), in this case the Queen Mary Lover's Knot tiara



You know how people would get pissed off at a wedding guest who turned up in jeans and a tshirt? And no, I don't mean those sorts of weddings where the bride and groom just go "Wear what you want! We just want people to be comfy!", I mean a formal wedding. Depending on how formal, the guests might even be judged on what colours they're wearing, what length of dress they're wearing, how they're cut etc (if you go to a church, do you have bare shoulders? are you wearing a hat? etc). The sort of resentment a wedding guest might feel for turning up to a wedding in jeans and a t-shirt is amplified 100,000-fold for someone like Kate. There's specific rules of etiquette around the sort of outfit, they're always very fitted. Look at Camilla -

there's not much scope for a billowy loose casual number that would obscure the waist. Even her billowiest Jenny Packham gown (and ironically this is also a Jenny Packham gown) was structured at the waist. Dressing otherwise would be seen as a snub to the assembled dignitaries and her entire job is to be pretty and have great hair and walk around in gorgeous gowns that are the correct-level-of-formal at various state functions, and she could maybe get away with something more like what Camilla's wearing but then the press would have her under the scope for dressing "matronly". Her
maternity dresses were structured.
There's certainly an element of
"why not just not wear the fucking onion?"
but despite Charles's desire to "modernise" (relatively speaking) the Royal family (because he grew up in the true old school, and found it restrictive and obstructive) I think Will very much seeks more of a RETVRN, possibly because he realises if he doesn't wear the onion, there's really no point to him at all. There's likely also an element of seeing what his father did wanting to shrug off tradition and modernise and seeing his brother embrace that which has spooked him off, and Kate no doubt is on the same wavelength. Supposedly that's how she ended up wearing that elaborate Alexander McQueen headpiece at the coronation - because Charles and Camilla banned tiaras, to try and "update" the coronation".
With all that in mind, you can hardly blame the woman for wanting to hide out for several months if she's finding the stoma a lot to deal with or has otherwise had a drastic appearance change post surgery - but it is also understandable that people started thinking there's something more nefarious at play, given how unlike the royal PR machine this has all been.