At low percents, it could be better than Plasma Whip, if the latter is rather decayed. You'll never realistically be able to charge the fsmash the whole way during a stun (if you can, you're doing something very, very wrong). Furthermore, only the closest part of the fsmash can be charged, and it will probably only get to 13-14% and a tad bit more knockback than usual, which isn't really all that much. Plasma Whip does 16% fresh and around 14% with one instance in the move queue. At that point, it'll still have better knockback than a fresh fsmash, I believe.
Regardless, even at low percent, following up a dsmash with another dsmash then an aerial is almost always better. For instance, the first dsmash (assuming it's fresh) will do 11%. The next one will do something like 10%, and then if you follow up with a nair, that's a 31% combo, as compared to a 24% combo. You could also just do one dsmash, follow up with a SHFF uair into a Plasma Wire spike to a utilt. That's a 21-22% combo right there, if your uair has been used once or twice.