"Ugly can be beautiful; but pretty, never."
I've received a couple of queries from confused readers, so I think I ought to offer what clarification I can. If we were to grammatically expand the assertion, it would read like this:
"Ugly can be beautiful; but pretty can never be beautiful."
This is to say that what does not follow the popular conventions of enjoyable appearance (ugly) can still possess something deeper beneath the surface which speaks to the soul (beauty). That whose purpose is to convince the viewer that its function is in its appearance (pretty) cannot have the deeper possession of beauty.
That's about as far as I can get without tripping over the fact that the expression itself has a certain inexplicable aesthetic. If the poetry is ruined in the expansion, then I'm sure there will be another unrelated post following soon.