People think that for someone to be overrated they have to be a bad player that people think is good. That is not the case, good players and even great players can be overrated, case in point Auston Matthews.
Matthews is an elite goal scorer, best goal scorer in the game, but yet he is incredibly overrated. The Leafs have a 39-15-2 record when Matthews is OUT of the lineup, that's not good for his 'impact' on the team, Matthews scores at a 72 points per 82 game pace in the playoffs, not terrible, but not worthy of being placed in the same tier of players as McDavid, MacKinnon, Kucherov etc..
The best players in the game, elevate their play when the games matter most, for 8 seasons Matthews has proven to NOT be that guy, I'm not saying he never will, he may figure it out much like a player like Datsuyk was criticized early in his career for not being a playoff performer until he figured it out, but until Matthews does that, he does not deserve to be in the conversation with the best players in the game.
That's the definition of overrated.