People hate him because they are used to cliches and Tidus character IMO breaks away from the mold.
He's not the broody bad ass hero neither he is the cliche type of the weak geeky boy (that will eventually become Bad Ass.)

Tidus is already a "Bad-Ass" in his own world since the beginning of the game.He is the guy who enters in a stadium and thousands of people go wild.He is the biggest blitzball star player of Zanarkand which is perhaps the equivalent of Lionel Messi in today's soccer.
However unlike most stars in reality he's never arrogant throughout the game.His talent gives him a confidence which,combined with his cheerful nature makes him the embodiment of the carefree youth.
People say he is stupid when in fact he is just akward.And of course he would be akward,he travelled 1000 years in the future for Christ's sake!Today we travel to another country and we feel like aliens,can you imagine being in his position?
Yes he is not the brightest of the company but this also makes his character more believable.How many football stars are very intelligent persons?Very few,if any.When you devote your life to excel in one thing,(in Tidus case Blitzball) you have no time to practice more intelligent pursuits.This IMO makes him very realistic.

I love the fact that he doesn't think of things too much,he just goes along with an attitude of carefree innocence and the optimism of someone who is very confident in his abilities.