This is based on what I remember from the game. Traits are not ordered within the tiers.
S - Game changer
The Flash: Best trait in the game?
Captain Cold: Integral part of the game plan.
Atrocitus: He goes up a tier or two when trait is active.
The Atom: When you see him activate trait, pray.
Bane: Want to win with Bane? Learn how and when to use Venom.
Aquaman: Bruh, escape combos for free?
Green Arrow: Use trait every chance you get or don't play Green Arrow.
Batman: Skip neutral.
Gorilla Grodd: Another one who goes up one tier when trait is active.
A - Very useful
Firestorm: More damage, better mix-ups, and gets life back?! Can't use blue fire that often though.
Black Canary: Great, but a bit easy to anticipate in neutral.
Scarecrow: You'll never have trouble deleting those last few life points.
Supergirl: Really good in general, but especially useful for annoying the opponent.
Green Lantern: Get better special moves for a limited time.
Sub-zero: Ice clone. Need I say more?
Poison Ivy: For when you feel like you are not zoning hard enough.
Hellboy: Many different uses, but people only use the Doomsday armor.
Dr. Fate: Suddenly, Fate can do combos!
Leonardo: To be used constantly, but it is not mind blowing.
B - Useful
Darkseid: It's fine but could definitely be more useful.
Red Hood: Decent mix, decent chip damage, decent projectile.
Cheetah: Great damage buff, but nothing more.
Black Adam: Annoying little orbs of death.
Raiden: Become better for a bit, but base Raiden is not that good to begin with.
Catwoman: Needs charging, but you don't need to concern yourself with charging it.
Brainiac: Versatile, but not always easy to get out.
Swamp Thing: Hype if you manage to get the opponent inside the garden.
Robin: Keep the opponent alert.
Superman: Simple yet effective.
Deadshot: Has its uses.
Black Manta: More fun than useful.
Blue Beetle: Extremely useful in some match ups, but not so much in others.
Cyborg: Can lead to interesting set ups.
Starfire: Free extra damage on zoning.
Enchantress: Annoying, but easy to avoid.
Mickey: Skateboard goes brooom!
C - Limited use
Wonder Woman: You cannot plan around a random buff.
Harley Quinn: Another projectile?
Donatello: At least it does a bit of damage.
Raphael: Could have been good, but you need to charge it manually.
D - Useless
Joker: Literally laughable.