Sliders help a lot, but I've seen some folk do amazing work without them. When I think "pudding face" I think about how the sims would all look alike sometimes if they didn't have different hair and skin tones. Twallan and Pescado help out a lot with curing pudding face.
I am really bad with working with the sliders and making faces, so what I do is take two sims that someone else made and use the "make a baby" button a few times until I come up with something cool.
When I think of pudding face, it means a character with such blah, basic features that if 1) you ignored hair color, eye color, and skin color and just focused on the face itself and 2) bred them to another sim, there wouldn't be a single feature you could look at the kid and go "Oh, he got _____'s nose/cheekbones/forehead/whatever" about.
When I'm making my sims I always start with the hair that's tied up under a bandana and a neutral skintone, because it's easier to see how interesting or bland the features are without them.
Of course, if you're talking about the phrase itself, it's from the idea that their faces look like a round plain bowl of pudding.
Kewpie, this might help you. I usually randomize the sim until I find one with a particular odd feature that catches my eye, like a pouty jaw/mouth combo or a long nose, and then I build the face around that.
Snickerson: a Random Legacy Challenge. There are zebras involved. Zebras.