Well I've been told the reason Chewie looks so clean and well groomed by Rian himself.
Chewie has been on the island for a few months , where its a life of living in the open with roasted porg every night over the camp fire but damp and smoky leftovers in the morning . Theres nothing to do , he's bored without Han and it is, to be quite frank ,taking its toll on him.
Even the interior of the Falcon is getting more filthy from all the mud and ashes he's been trampling in everyday (why do you think its always looked like a dog has shaken itself up the walls for forty years). He's caught fleas from the porgs, the sand is getting everywhere and in the cockpit where he sits and cries over Hans empty chair every day his lost hair litters the flooring. He feels alone and unloved.
Reys gone off alot with Luke now (who clearly blames him for not protecting Han) but last time she came back she went all darkside over the state of the Falcon, and how she doesn't have to clean it up just because shes a girl ( even if Kylo Rens Force nick name for her is " Mary Sue" ), and if Chewie doesn't sort it out she'll skin him alive with her sabre and turn him into an interesting piece of furniture for the Falcons lounge area.
So for Chewie , still depressed over the crap way Han died and the fact old age is affecting his eye sight so badly he didn't manage to shoot Kylo anywhere fatal,hits rock bottom. He grabs a mop and bucket and wanders down to the sea to get fresh water.
As he glances down at his reflection he sees how different he looks now,old and tatty and is overcome by a fit of depression . There and then he decides to throws himself in, intending to end it all. Afterall Leia didn't hug him , Luke has clear gone mad and never visits, Reys a bitch and eatting so many porgs has given him gas so bad that if he farts by the camp fire he's indanger of setting himself alight.
But as he wades out into the ocean to drown, some porgs start to follow him. Actually they're more interested in all the sand fleas that are jumping out of his coat on little fur life rafts ,which are attracting the fish ,which the porgs scoff down. Chewie yammers at them to leave him alone so he can drown himself with dignity ,only to discover they're really good mimics and can talk back to him.
Chewie remembers Hans deep love of Wookie show tunes and sings the porgs Hans favorite song as a lament to his lost friend and the porgs are so impressed they join in with the chorus. Chewie realises he is the fabled "Chosen One" , the "Porg Whisperer" the caretakers of the island have been waiting for eons, not all these stupid jedi tourists that keep getting bitten by the sand fleas and catching a rare midichlorian disease that gave them alot of difficult personality issues to go along with the wierdly unpredictable raw powers.
Refreshed and with a new purpose to his life Chewie swims around the island trailed by flocks of singing porgs and he swears never to eat another. And when he makes land, the porgs follow him upto the Falocn, where in a touching and funny dance sequence ala Mary Poppins, they clean the ship together.
And when Rey returns shes so impressed she offers to give his fur a bit of a trim with the lightsabre (for the first time in forty years) and (with all the porgs hes eatten being so fat and juicy) Chewie realises his fur has actually taken on a special luxuriant shine that any Wookie over two hundred years old would really want.
So after another swim and a conditoning blow dry from the Falcons engines he gets Rey to lend him some of her jedi hair care products and gets R2 D2 to style it in for him with all the extra grooming tools Luke hid in him years ago ( when he knew he would need a good barber to loose that really wild " lost in the wilderness look" when he returned).
Chewie looks at himself in the Falcons mirror. He looks fabulous!!! Hes happy and content and smells great mainly because he's stopped passing porg driven farts every five minutes. Life is all good again.
Right up to the moment when the First Order steam back in ,and, after stealing Rey again (Oh boy, I think that young lady's gonna have kidnapping and torture issues later on in life to go with the "!Mary Sue complex"), they toast Porg Island .
And now Chewie IS pissed. Hes going to make the First Order pay.
The really story in the film is not about any "Last Jedi. Its "Porgies: Chewbaccas Revenge"!!!!!!!
And hes going to be well groomed about it.
PS This is for The Wook. Like everything else