I think that the "culprit" with Ingrid wearing her mother's clothes is that younger people are very invested in sustainability so wearing a dress of their mothers is their way of addressing climate change and other environmental issues. I think the same applies to Danish Princesses Isabella and Josephine.
Speculation on my part, I suspect if they were to splurge on new clothes or invest in tailoring, that the princesses would rather spend money on clothes worn to off-duty social gatherings than for official royal events. In other words, they'd rather look their best for their friends / peers than for the public. Although I will state on my end with Ingrid, I think that she looks fine when she wears her mother's clothes to daytime events - yeah I can see that the clothes are dated, I did not need to be told that the daytime outfit was a Mette-Marit hand me down, and if I really scrutinize, I can see the fit issues.
Her clothes for evening events are where I have gone "yikes" a time or two, specifically this state visit and Christian's 18th birthday, and also an outfit she wore to one of her 18th birthday events that may have been Sonja's. It wasn't quite a "yikes" but the proportions were out of whack. Despite me having a "yikes" reaction, if Ingrid and other princesses are standing on principle, I respect them for it.