I think the lessons to take away from Marner's diminishing stock in the eyes of fans are pretty straightforward. First lesson would be to not make ridiculous contract demands, but if the player and agent are so inclined they should shut up about it and keep quiet like Nylander's camp did instead of using media mouthpieces like Dreger to try and drive up the price. Also would help to not immediately start lobbying for a big payday on social media not even a day after your supposed buddy just signed his deal.
Second would be taking responsibility for poor play when you very clearly didn't have it, it's not cute when you underperform in the playoffs and your answer in the media is "naw I wouldn't change anything about my game." Granted this is an issue with the entire core who keep repeating the same lines about "lessons" and how they're "right there" despite all evidence to the contrary.
Third would be to show some modicum of fight or pushback when you're very obviously getting bullied. No one likes watching a pacifist, especially in hockey. You don't have to drop the gloves but no one wants to see you smile and laugh after taking a punch in the mouth or getting face-washed. Matthews had the same problem but he's better about it now and is more willing to engage physically and not take any guff. Mitch still plays like a coward who's afraid of taking a hit.
Of course all of this would be ignored if Marner showed up in the playoffs like he's paid to do and actually produced like similar stars in this league. The fact that he doesn't makes all the aforementioned issues more glaring. Fans would be willing to give him more grace though if he didn't make his negotiation a circus, wasn't overpaid, and was more willing to take responsibilities for this teams many failures, especially since he wears an A.