Pa...Pancake faces?
Ume-sensei's widefaces are awesome and adorable. I guess it's something of an acquired taste.
As for whether they fit the mood of the show... I think people are too quick to dismiss this anime as a dark/fucked up show simply because Urobuchi is involved in it. It's obviously not all going to be cats and rainbows, but I don't think it'll be quite as bad as people assume. Sure, Urobuchi has wrote some very messed up stuff before, but that doesn't mean he's unable to write something different and a little more light-hearted.