Teemo is useless in the late game. He's too squishy and has low crowd control. He'll just get burst down. The shrooms aren't much of a threat if you've got a healer on the team, teemo is dead, or smart enough to try to predict that shit and let the tanks take the worst of it.
Useless is a bit of a misnomer.
The reason a lot of people think he's bad late game is because his ultimate doesn't work like a lot of popular champs retarded *PRESS BUTTON FOR INSTANT GRATIFICATION* ultimate abilities. Teemo's shrooms are actually really useful for team fighting, especially with max penetration, it's up to you to place them well ahead of time and for your teammates to play around them. If he farmed well his Q will do MASSIVE damage too and the blind is the natural enemy of an ADC and most assassins in team fights. And end game his passive with a sweeping lens is extremely useful too, if you're good at positioning.
Teemo is a lot more technical than people will ever want to give him credit for.
Whenever his shrooms are off cooldown there's no reason to not just drop it nearby, never hover 3 stacks, because you're wasting shrooms.
Even if I'm in the middle of lane I'll just drop it a bit back, so I can roam and still get some CS and make it a bitch for people to push my lane. Come the end of lane phase if I spend a lot of time defending mid sometimes I'll find that I've got 4-5 shrooms running down our side of mid lane. Even if a lot of my team gets killed, when the enemy team goes to push mid their wave dies and usually they take a ton of damage, completely stopping their momentum.
Which is a pretty sizable team contribution actually.