English term (edited): on [a] diet

I think you can read it quite literally here.
However, do note that there are other clues from the overall phrasing to the writer's views here:
"Some not on diets" carries a potential implication that many or most of the others would be — i.e. women who were watching their weight. So the writer might be implying "some of the women were a bit fat", while also perhaps expressing the idea that it was standard practice for these young women tp be on diets; they seem to be implying that the slightly older women were perhaps the ones not bothering...
Overall, the writer is syaing that some of the women were attractive (and hence worth observing), but others were less so.