a 10s oracles on a 120 second cooldown is not a valid counter for his shrooms. get that into your head. he can place twice the amount of shrooms than i can clear in that short time window.
to remove those shrooms you have to have the opportunity to properly sweep them. you cant do that the moment you want to make a move on him, when hes sitting in a pile of shrooms. and hes ALWAYS sitting in a pile of shrooms.
next problem is, the trinket only allows you to remove more than 1 shroom after level 9, when he already has had time to litter the field with those things. since the upgrade isnt available earlier.
pink wards are a pretty shitty mechanic to get rid of them aswell, as you can a) only place one, b) he has range auto attacks which makes it relatively easy for him to sneak in a couple attacks on the ward even when youre trying to defend it, and c) they are expensive and you effectively feed him 30 gold every time you want to clear out those things.
also, the hate towards teemo isnt because of his viability. hes not overpowered, we all know that. but he is so anoying, and his design is so toxic, that he makes people want to ragequit at the loading screen.