I think it was an intentional artistic experiment. Can we make a character who lacks something basic in our understanding of a person's feelings, and still have their emotions 'read' to the audience solely through performance and characterization?
This goes back to the 'less is more' axiom. The less specific details you give to the viewer, the more you engage their mind as they fill in certain blanks for themselves. And so it becomes far more personal to each person.
One could argue that it's just an extreme example of the earliest Muppet designs, with their abstract features and simplified shapes. The less 'literal' something was, the less defined, then the more people it could reach and resonate with.
DN