So classic cartoon art, basically?
The trick about good "flat" shaded art with thick out lines is that you need a very deep understanding of how the shape works in
3D! Because outlines is all you have, really, those lines have to be
perfect. For example, even a shape as simple as Mickey Mouse has 3D geometry underneath it. There's at least a sphere and then some more complex ellipsoids forming the nose:
You can't just draw "a nose" and "a mouth", you always have to consider the whole body, perspective and proportions otherwise you end up with an unironic version of these "Dolan" comics.
That "flash style" usually just involves using rather thick outlines and variations in thickness to create a hint of light and shadow. The Binding of Isaac characters tend to look like simple doodles (and they are, for an experience artists) but they always keep a certain amount of perspective depth. Look at the shading in this picture, the way the mouth of the Duke of Flies is modeled or the way the shadow wraps around Isaac's eye, you can't achieve such a look without a feel for how those characters are shaped in three dimensions: