No, that's the band they wear at the feather's base to keep it tidier then it would otherwise be.
Maybe those feathers
are natural, and appear due to the hormonal imbalances of getting older/more badass, in the same way a human might get hairier (or less hairy
) as he/she gets older.
I always thought of mudokons as belonging to a vertabrate calss other than the five we have here (mammals, birds, reptiles, amphibians and fish.) The same goes for most of the other creatures with bald, brighly coloured skin: scrabs, paramites, glukkons, sligs, slogs, slegs, outlaws, wolvarks, fleeches, possibly slurgs, Vykkers and Interns. The others are all mammal-like, bird-like or amphibious.