Nope. The writer decided to have Diana not kill Superdoom by deliberately pulling back her strike. If he wanted to show that Diana was weaker then Superdoom, he could've just had Diana's strike bounce off his neck.
Diana stabbed Doomsday through the chest with her sword. It did damage. It stopped Doomsday in his tracks. Yes, the sword was destroyed and Doomsday healed the damage. But damage was done, nonetheless. A strike like a decapitation, however? I doubt Superdoom will just pick his head off the floor and put it back on his neck.
Darkseid's supposed to be an even bigger threat than Doomsday. Saying Diana's sword can't just decapitate him still makes sense. And even still, we saw Diana's sword was strong enough to take out one of Darkseid's eyes. Stands to reason it could do more damage as well.Otherwise, if Diana is powerful enough to wield her atom slicing sword at opponents like Doomsday or Superdoom and be able to cut off their heads, she would have an instant type of solution against other universal threats like Darkseid. I don't think the editors or writers would let this happen, even though it would be logical given her strength and an apparently magical sword that can slice atoms.
If the writers and editors don't want Diana one-shotting villains with her swords, then all they have to do is make sure Diana doesn't manage to land a killing blow with her swords. They don't want her to kill Darkseid? Have him blast her before she connects. Have one of the Furies tackle her. Whatever.
But Diana injured Doomsday once. That scene with Superdoom built up all the tension in the world until Diana pulled back at the last second. Diana, armed with one of her awesome swords, is a threat to just about any villain. I like it, myself.