Idols are athletes. imo if they really want to be the best that they can be, being fit is one part of what they need to do.
Fit doesn't mean super thin. They are many athletes with big builds.
It is the companies responsibility too. When there is the culture where idols want to be fit and have a healthy diet, they are less likely to get injured or suck.
I'll take the example of Loona. During their US tour, they were dropping like flies. Although all are pretty and thin, many of them were not fit.