Do you even lift, bro?

It depends mostly on story and what I need for individuals as to exactly how things go, but most of my played characters do pursue fitness activities. If a sim absolutely must be fit to stay in character, I usually just cheat a flag to freeze their physique because my fun doesn't come from making them work out. I'm too busy with other parts of their story (and many other bits of story involving many other sims) to do it as a game-mechanic thing for every sim where fitness matters. Of course, in cases where the sim's life journey is the primary basis of the story, they have to work out the usual way (or not) and be either fat or fit or somewhere between, however it happens. I do tend to encourage many of them (sometimes even certain unplayed townies) by giving them the Gym Rat trait either by earning it or by cheat.

Edit: Also, I also flag sims for freeze physique when I don't want them to get too buff due to story reasons.

I've tried the Eco NAP choice of Fitness a couple of times recently. It's supposed to encourage community fitness and other healthy choices (like eating). I haven't focused too much on its effects on NPCs though.

I wonder if it helps keep everyone in decent shape without creating extreme body-build results. Does anyone know if this Fitness NAP helps moderate sims' muscle/fat ratio while still letting them be themselves?

Some of my sims are really into fitness and some arent , it really depends on the sim I'm playing, I always start off my sims being slim, I don't care most of the time if they put weight on though

I've never had a problem like this because my sims, when I play, eat healthy. They never gain weight. If I make my sim work out it's for a reason.