After you will have stepped on a fed teemo's mushroom once, you'll understand why everyone groans when they hear teemo.
also, in team fights, he's pretty much as useless without a good team as Kog'Maw. He deals a lot of damage with little items, but his auto-attack range is almost melee.
He basically is a counterpick for multiple solo-top champions and kind'of fails at anything else if the enemy team has proper ward coverage and an oracle's
I don't like a teemo in my team also because he's so squishy for a solo top, and bruiser teemo feels....odd...