Why do people think Kennen is bad?

Because he is bad. His only purpose is beating bad players. He's borderline useless at higher ratings.

He can't 1v1 any common top, due to low burst damage. Almost everything in top lane will kill him before he kills them. Many champions, like Riven, get a guaranteed kill if they manage to touch him, which is very easy for skilled players to do, due to his low auto range.

Many top champions can sustain through all of his damage, even if he lands every shuriken.

His mobility is poor. Any slow, stun, knock back, or snare will invalidate his ultimate, and most champions have a way to jump out of it.

His survivability is poor. If he gets stunned trying to get in range of your team, you can likely kill him before the stun wears off, or force him to Zhonya before he can get into a good position.

If he rushes survivability aside from Zhonya, he does so little damage that he can be ignored.

If he rushes damage, he dies almost instantly, as he needs to get into the middle of your team.

Overall, he just has no place in the current state of the game. He was good before mobility and CC creeped up, and when team fights lasted more than 5 seconds.

I say no damage late game.

People focus on what is optimal. Kennen does not do optimal damage late game, and when attempting to get said damage he ends up lacking optimal survivability.

There isn't a single champion who can't destroy in lane right now, however it takes a fair amount to an extreme amount of personal skill to make suboptimal champions work against optimal ones. If you get them working, however, you'll have a considerable advantage over people who never learned that matchup.

Basically optimal champions take a certain amount of effort to do well with, where as suboptimal champions can do well with a significantly larger amount of effort, but put additional unknowns into the equation that end up favoring you. It's honestly worth it when you get past the learning requirement.

I personally think Kennen is crazy strong and never underestimate him

Well, Kennen is crazy strong, but he is super hard to play since his only viable way of dealing AoE without using his ultimate is using his E, and he is squishy. Also his Q is easy to block. Kennen is crazy strong if you have a good team comp and can play him, but those that can't suck at him.

His all in is bad unless he gets a stun, which can also be hard to proc unless you land your ninja stars. Other than that kennen is good. Ult +

It has been about a year since he has seen serious competitive play so I imagine there are people that have dismissed him as a viable champion, forgotten what he does, or both.

Honestly I really like Kennen. I like champions that can be played in different ways, sometimes I'll play him as a mage, sometimes ADC. He's squishy but you can build Rylai's, Liandry's, etc for some health. He's a great initiator, E in, Ult, Zhonya's and the team has taken some massive damage. If your team follows up, you can likely pick some off with your W once Zhonya's is done, too.

I don't really see people complain when I play Kennen as a mage, but when I play ADC they think it's a troll pick. He's got good attack speed, and his E's passive works great for ADC Kennen (Every 5 attacks, Kennen deals bonus magic damage equal to 40/50/60/70/80% of his attack damage and adds a Mark of the Storm to his target.)

I was just raped by a Kennen, playing Talon. He is a extreme lane bully very good against Talon and any other manan based assasins. I don't know about Zed since he can poke just as hard since he has energy too. Kennen against any AD assasin is good since he just needs to rush seekers armguard and do massive damage early.