It's actually just normal diffuse shading, but the way I've set it up is that there's 90% ambient light and only 30% from the actual light source to give it a slight shadow, and the combination of the two adding up to over 100% makes the colours all bloomy like that. I just set the intensity per scene depending on what I think suits it best at the time.
I used the same lighting technique with
this render of my Spike model (with 75 and 50 for the intensities), but I also added an outline that follows the camera to make it look more fitting with the show as well as fitting with KP-SS's renders of his own MLP models. I guess you could sorta consider that cel shading in a sense.
The outline on Joan was actually an accident, probably something to do with me rendering the image really big and scaling it down while keeping it transparent.