I'm always amazed that such an image-conscious, portrait-loving royal family as the DRF they don't take better photos of the kids
Me too - but I think it is good and bad at the same time: Good: they are not used as props like in other royal families and the pictures seem "more honest" to me - they look like many teenagers that you pass on the street, no efford, no dressing up, no staging. Innocent snap and in a decade they will shout "did you really release a picture of me in track suit bottoms?".
Then within that very casual surrounding a few thoughtless things have happened that are lightheaded because these pictures have an entire different scope than "normal family snaps" - the picture from this summer where Josie's underwear was in full display come to mind. These pictures might be okay in that casual family surrounding, but not for that purpose.
Bad: Christan and Izzy are old enough now to realize that they are not normal kids. They profit from being very privileged. A certain dress code is part of that game. They should learn now.
Plus: It marks, as the post points out, a very sharp contrast to Mary. Frodo also -despite wearing appropriate clothes -very often looks as if he just rolled out of bed and despite being in good shape has yet to find a suit that actually fits him (can it be that hard).
In every other upper middle class family grandparents etc. would not be too delighted to get a pic like that (as a snap, okay, but not as an official picture). A lot of grandparents expect some "proper" pics of their grandchildren - including suitable clothes. That should also be the case here.