As a Janna main, she definitely has a few annoying tools like her ability to point and click cripple you / outrun you at her leisure and knock you up when you chase her, but that's not what people really hate. It's her shield, and to a lesser extent, her ult. People hate Janna because they despise that Janna can be so beneficial to her lane by just pressing E. There's nothing enemies can necessarily do about it. Your damage does nothing, and their adc does more damage just because Janna pressed E. Because of that defense / offense combo on a low cooldown, and with Ardent being as strong as it is, Janna can be played by a monkey to some effectiveness so long as the monkey knows to move away from the enemy team and press E on the adc.
Her ult is also a very long instant knockback that she doesn't have to aim, which absolutely shits on some melee champs. Point is, a good Janna can do a lot, an inexperienced Janna can be very useful regardless. Is it as bad as everyone says? I don't think so. You have Lulu that has a low cooldown shield that supplies damage from Pix, a point and click nope spell, a massive slow that's IMO easier to hit than Janna's Q, and an ult that peels for the adc and pseudo heals them. People don't complain as much about Lulu anymore although she seems like she should almost be worse, especially because of her passive allowing her easy harass early.