Overrated: Earthbound
I have tried multiple times over the last twenty years to get into this game and understand all the hype it receives in nearly every corner of the internet. There are people who seemingly worship this game and will tell you with a straight face it's the best JRPG of the 90s, or at least a top 5 RPG on the SNES. The game has some serious competition on the SNES, and it's hard to argue that Earthbound does anything better, or in some cases, even half as good as many of its rivals on that console. I always find this game boring, bland looking most of the time, pretentious, and just annoying. I feel like a group of people who really, really loved this game built up its mystique on message boards starting in the late 90s and into the 2000s, and by the time retro game collecting began to gain traction in the later part of the 2000s, this game's reputation as this amazing JRPG had outgrown the actual merits of of the game. Whenever I say this I always have a lot of people disagree with me, but I feel confident in calling it overrated given how many times I've genuinely tried to understand this game and its appeal, and failed each time.
Underrated: Resident Evil 6
There might be a better example I'm not thinking of, but in terms of games that get a ton of unwarranted hate and are often looked down upon (unjustly imo), it's hard to think of a better example than RE6. I actually expected to hate this game as much as 90% of most other people online when I first played it, but in the end I liked it more than even some older RE games I've played. Yeah, it's essentially what you'd get if Michael Bay directed an RE game, but for what it's worth, the game has a great story, excellent visuals, cool bosses and enemies, and pretty tight gameplay. There are some annoying aspects to the game and it's honestly a bit too long, but it's hard to fault it in many other ways.