That last goal for the fitness aspiration, "maximize body potential," means the two main sliders in CAS are maxed right out-- Fitness all the way up, Fatness all the way down. It isn't hard to make Fatness go back up, but the muscle definition will remain. Avoiding activity for a long time will let Fitness start falling back down, but it's slow. Very slow! And your sim's spontaneous pushups and situps go a long way to delaying it.
So, what can you do about it? Cheat. Ctrl-Shift-C, cas.fulleditmode, and then go to a mirror and "Change Sim." Now you can pull the Fitness slider down until the muscle definition fades to your satisfaction.
If you're a mod user, you can find a skin overlay you like, and those almost always smooth out muscle definition. On The Sims Resource, RemusSirion's most recent one will smooth out the bulging muscles, but won't cover up tattoos, and your sims can tan normally.