Personally I don't like the terms Overrated/Underrated. While I use these terms, I use them to mention how surprised I am with the popularity, not that I think it shouldn't be like that.

If a game is too popular, there is a reason for it: Great Quality, Timing, Marketing, Emotional connection. The game will feel overrated when it checks these points for noticeably bigger amount of people.

It's not because it was bad... It just feel that compared to a game that has more loyal fans, the game has unexplainable popularity.

Of course there are overrated games that are completely bad but those are very rare cases, and people usually start to see that in few months at most and won't feel overrated anymore.

Also, the mainstream fans and the hardcore fans like & demand different things of the same game/franchise.

A game that is popular among mainstream fans, will always feel overrated to the hardcore fans, but in terms of 'pure' business, these are the games the company should make more of. They are more profitable, especially in short term. (Though, mainstream fans aren't as loyal so when the trend ends or get saturated, every game using it will start to feel bad. I'm looking at Sandbox/Open World when I say this.)