Most people (the people who don't play him) hate Teemo.
Teammates hate him because he doesn't add much to teamfights and is squishy. They underestimate the vision control, CC, and damage his shrooms provide.
Enemies hate him because he's the "annoying fly" of laning. He whittles down any enemies that can't kill him (the ones that don't outrange him or have no gap closer and damage abilities), and he's practically ungankable after level 6. He's a good splitpusher, too.
Everyone "hates" Teemo because he's one of League's most iconic and lovable characters, but he's also one of the most painful to play against.
I wouldn't call him a "troll pick" (there are no champions that are entirely unviable, in my opinion), but he doesn't have many good matchups anymore. The few top laners that he's good against (again, no gapclosers or strong poking abilities) are fairly rare. He struggles against mages and strong all-in champions that don't rely on their autoattacks too much.
If you master him, he's helpful in his own way against any enemy team (mushrooms and splitpushing), but he's really vulnerable in lane. He doesn't add much to a team besides his mushrooms (which got nerfed very hard several months ago) and AP DPS.