It looks like the heat from the transformer is "cooking" the glue or whatever is wrapping it, over time. Whether that is bad or not, I can't quite say. I've seen similar issues before, ocassional, after taking apart dead PSUs and other electronic gizmos with transformers in them.
I would ask one question though - is the transformer getting too hot? How is the cooling? Should it be heatsinked? (Beware of electrical issues though.) Perhaps some PSU guru would be willing to chime in here. I've seen it before, but I can't say whether or not it is some sort of indicator of potential future failure.