...your complaint is that Teemo, who had among his originally intended roles the words 'anti-carry', did in fact beat the stuffing out of a pretty heavily auto-attack reliant AD carry, who had foolishly seperated from his allies and thus had no one to peel for him?
Well hallelujah, for once there is an aspect of Teemo we can stamp as 'Working As Intended'.
In other news 750 damage per second sounds excessive. If he could do that I think I'd play him more, so let's do some math.
Teemo's shroom has, at max level, a base of 112.5 + 0.125*AP damage per second.
Now let's not forget Liandries... That's 4% of Yasuo's max health per second as well.
So our expected peak damage equation from the shroom and Liandries is:
112.5+0.125(Teemo's AP) + 0.04*(Yasuo's Max Health) for the first 4 seconds, then 0.04*(Yas' Max Health) for an additional 3 seconds.
Now, I'll just make some assumptions here. 750 / s is big numbers, so we'll say it's late game, and assume both parties are level 18.
At level 18 Yasuo's baseline health should be about 1912.
For his items, I seldom see Yasuo build much defense, and most commonly I just see resistances.
Nonetheless, we'll assume this Yas was odd, and, to bump the numbers up, I'll assume he purchased his sole tank item as straight health ,and purchased a Trinity Force and a Warmogs, for a total bonus of +1050 and a final health of 2962.
So now we see what numbers Teemo would need to hit 750/s.
750 = 112.5 +0.125AP + 0.04 * 2962
750 = 112.5+0.125AP + ~118.5
750 = 231.0 + 0.125*AP
519=0.125 * AP
AP = 519/0.125 = 4152.
In the scenario you describe, to do 750 damage per second, before counting MR, Teemo would need over 4,000 AP.
Now, son, I have to ask.
Are you sure that Teemo wasn't fed? Are you really totally sure?
tl;dr
Stop throwing a bitch fit, Teemo can just about manage to lane bully those who meet none of the following criteria: Has a gapcloser& stun, Can engage him from range without relying on auto-attacks, can assault him from outside his Q range. After laning phase is ended he only gets to contribute to a teamfight if the enemy team decides to helpfully host it's teamfights in shroomed up areas. He needs changes, and badly, but straight up nerfs are not what you're looking for.
Additional tl:dr
Do not spout bullshit which can be easily checked via basic math number wizardry
