Good counters are lane bullies or people who can kite him. Try to pick strong early game ADCs, and if you're the Support pick hard CC or disengage.
is good due to her natural high range, nets can make it impossible for Morde to reach you, and she's well known for her traps being abusive against melee champions. Her traps make or break a good Caitlyn, especially against Morde.
her strong, low cooldown poke is good since Morde is vulnerable to poke, but it also has the same range as Caitlyn's auto attacks so it can be used for zoning. One thing to keep in mind is that Miss Fortune can all-in very easily unlike Caitlyn, Caitlyn relies solely on poke but Miss Fortune doesn't, so if you're ever in a situation where you're being all-in'ed by Morde, don't run away because you might be able to win especially with Miss Fortune's absurdly high early game damage.
is great not because of her range or anything, but because of her traps. Her traps are easy to use and waiting for the snare to weare off, hitting Morde with your Zap, and continuously bombarding him with auto attacks seems effective.
Graves got nerfed hard but he's still great in a lot of matchups like against Morde. Using his Smoke Cannister will ruin Morde and you can reposition around him with your dash. Try to zone Morde off with your poking abilities and since Graves is fairly tanky with high base damage, he can take a few hits and afford to throw some back.
her disengage/peeing abilities are so good against Morde, don't use her Tornado until he's up and close to your ADC and try to save her slow for if he's running away or still trying to engage on your ADC, that way he doesn't immediately back off and flee. Her shield also can be an issue since Morde relies on burst.
just headbutt him away. that's pretty much all there is to it really.
can be risky, but with a good ADC your ability to lock him down and Leona's shield lets her be annoying. Morde might try to blow up you as Leona, so be careful.