People who play Teemo like they are trying to play a ranged carry are doing it wrong. Most people lack the imagination to understand and implement Teemo's true role, that is, of map control and mobility. Spec for high CDR, buy lots of wards, and run around the map a lot.
At level 6 you should plop some shrooms to help your lane partner and by level 7 you should be roaming and establishing map control. Whenever you see an enemy trying to get a buff, harass them away from it and steal it. Whenever your opponents think about ganking, intercept and divert them.
There is no point in trying to carry with Teemo, but having him be support/dps can really contribute to a team.