Nothing is wrong with it. It's just not your tastes.
It's not overrated because honestly the views on it are completely mixed. You either love it or hate it. Not overrated. Not underrated.
Personally it's my favorite Zelda game. The atmosphere and characters were just mind blowing at the time. A lot of the NPCs are from Ocarina, but in this one they actually feel more alive and are given way more personality. They actually had schedules they stick to and jobs to do. It was a unique experience for N64 players at the time.
The masks were great and added more to puzzle solving in addition to the normal items. Rolling around as a Goron and swimming super fast as a Zora was a great experience for Zelda players who always wondered what it'd be like to play as them. It also made their respective boss fights even more fun to play.
The temple designs aren't as straight forward as in Ocarina. That's either a minus or a plus depending on the type of player.
The time mechanic seems stressful at first, especially if you don't know the song that slows down time flow in half (which the game never specifically ever tells you unless you talk to a scarecrow in a shop)