Yes Mizuki was weak, but it was a pilot so you shouldn't expect much from the earlier episodes. Kishimoto was still creating the "naruto universe." As for chuunins being weak, remember that most of the Chunnins are shown facing Jounins most of the time. You have Jounins face a Genin and they're dead meat. Look at Orochimaru v. Sasuke, Oro was just fooling around and no matter what Sasuke could do all he ended up doing was burning off his mask.

Edit: What makes the genins look strong is that all the rookie genins made it to the preliminaries, while people much older were unable to. This years batch of ninjas are exceptionally good and it makes them look like they're really strong.