Ehh, Link doesn't seem bad, but he has a lot of glaring weaknesses that really do him in and make him pretty much bottom of the barrel.
Link's pretty heavy weight and great MC move considering how heavy he is means he lives forever, but this means he gets combo'd/CG'd much easier too. Outside of a couple of characters who have ways to work around the downsides of being heavy (Snake, Wario, D3), heavyweights seem to be stuck in the bottom tier time and again.
Link's projectile game is good, and he can mix it up well, but honestly it's just outclassed by other characters' projectile games, and it can be pretty much shut down by a player who's good at powershielding and grabbing items in the air. Link is good at midrange, but being merely good isn't enough to compensate for how bad he is at other things.
His biggest weakness is having no close range/OoS game at all. Give him a 3 frame jab, a normal grab, and make upB OoS like 6 frames startup with half the cooldown and that right there shoots Link up the tier list a solid amount. Link's pretty much a punching bag up close, and it really hurts him. You can tell how crippling it is when you have to play a character that makes Link approach/is so fast they can get in easy anyway.
His recovery sucks, and certain characters (coughMKcough) completely exploit it. But it's not his biggest flaw. Marth and Falco both manage to be top tier with mediocre recoveries. Snake's isn't the best either. In Brawl you can usually make it back with good DI. That said, Link's recovery is close to the bottom, and that's just not OK when he's already got so many other problems to have to work around.
Link just doesn't have a lot going for him and too much going against him, forcing him to rely on outsmarting the opponent. Most of the ATs he does have end up being gimmicks with not a whole lot of practicality. He just not a good enough character to succeed without being a better and smarter player than his opponent. (I think he should be tied with Captain Falcon for 5th worst ahead of Jiggz, Zelda and Ganon, and I don't really see him being better than anyone else).
So basically, Link sucks, if you wanna succeed with him you need to be a good player, work on being smart, don't waste time with the silly ATs, learn lots of characters to make you better in general/ help visualize the MU, get a secondary to cover his worst MUs.
/Linksucksanditmakesmesad