I've been wandering around looking for all the lore I can, and while looking at Tahm's lore page I discovered that his list of Rivals includes all the champion yordles as well as Fizz. His only ally is the Darkin, which is also intriguing. The lore implications! ^^ On second thought he doesn't really hate them but still still still ^^
Yordles to him are like horderves served on a lightly salted cracker. He likes someone with POWER. Someone like Kindred, Veigar, Syndra, and bunches of tanks like Sion, Mundo, Cho'Gath etc. To him they're like a large King Blue Crab salted and peppered steam to perfection, with a side of garlic butter. And a glass of 100+ year old Brandy Red Wine. Mmmm I'm making myself quite famished. 
There's a headcannon running around that Tahm hates most yordles because for the most part they are very hard to corrupt and won't accept his bargains.
The only yordle he's ok with is Veigar, a corrupted yordle.
He dislikes anything that he cannot make miserable before eating. Yordles are simple and happy on the most part. Note that he doesn't dislike the miserable ones (pre-rework Amumu and Veigar).
Not enough meat on their bones.
I like Tahm more and more.
He dislikes Yordles because when Yordles stick together they're very... Spiritually resilient, for lack of a better term.
Did you notice that isn't on that list?
Ofc he hates Yordles. They are not enough to quench his voracious appetite. 
I though he didn't like Yordles because they were technically some kind of fay spirit (like Gnomes in D&D) and were sort of beyond his temptations.
You think poppy will accept a deal from kench to find a worthy hero?
You all have "good" points! XD But what Tahm actually eats is misery. Does this just mean Yordles are not miserable enough? or something deeper?
