Pretty simple. It all started back in Season 1, where his shrooms lasted 10 minutes, did alot of damage. If Teemo was fed, a single shroom could instakill anyone with no MR items.
Slowly as the hatred grew because of the players inability to build and counter Teemo smartly, he became weaker, getting nerfed over and over again.
Six season later, the hatred still exists like an abused crappy meme and players STILL complain about it despite Teemo barely being in the meta thanks to the buffs from the mage update, which made him BARELY playable in high elo up to this day.
And even when Teemo isn't that much of a threat, people would still rather see them dead or gone than admiting not being good dealing with him. And even those who know how to deal with Teemo still think it's annoying, which again is false.
The fact that Riot feeds this bad meme just makes it worse, both in crappy skins like Omega Squad, facebook posts about giving players the chance to delete him (Which failed, thank God...) and making some evil reference in the patch notes.
TL;DR, only idiots hate Teemo, a champion just like any other, due to the ammount of toxicity in the comnunity.