I agree that the default mood is kinda ugly, @
Celestia and I decided to change it to "Emotionless" instead. Sweetie Bot for the win.
Personally I don't feel that it's necessary nor a particularly good idea to have a "blank-space" mood. Moods were added as a feature to make people's profiles and postbits more fun and unique. It would make the postbit appear broken and non-uniform for people using it, as well as leading to confusion regarding how to change a mood if there is no image for one (I.e. you'd just see a random pencil-button thing floating there).