I would categorize the per turn damage categories into a grade of damage per 3 points of damage dealt, per tier of content. The damages listed below are assuming you are doing the damage total for the turn, so fighters or cantrips automatically scale, but other classes might need to use their other bonuses to get up to par.
F - 3(.5) Or less per tier
Acid Splash on 1 target
Attacking with a d6 weapon with no damage bonus.
D - 6(.5) per tier
Poison Spray.
Longsword attack with +2 bonus
C - 9 per tier
Casting eldritch blast on a target with hex
Attacking with a greatsword with a +2 bonus
B - 12 per tier
Eldritch Blast, hex, and Agonizing blast with +3 bonus
Attacking with a +2 greatsword with a +3 Str bonus.
Level 2 Magic Missile
A - 15 per tier
Eldritch blast, hex, agonizing blast with +6 Cha bonus
Attacking with a +3 greatsword with +5 Str Bonus.
S - 18 per tier
Attack with a +2 longbow, hunter's mark +3 Dex bonus, and Sharp Shooter.
Attacking with a +3 Greatsword, +5 Str, and raging for +3 damage.
Sneak attack+2d6 with +3 light crossbow and +5 Dex
Hitting 1 target with fireball - 28 damage.
So that's pretty good idea of how much damage per turn per tier is considered what grade (To me) keep in mind that at lower tiers, you can boost your damage per turn significantly with bonus actions. But they can't pickup as much slack in higher tier play as they don't scale up in damage as much as your standard actions.
Keep in mind dealing damage is only one aspect of the game. Using vicious mockery is going to do F grade damage. BUT granting disadvantage on the targets next attack could save the day.
F - 3(.5) Or less per tier
Acid Splash on 1 target
Attacking with a d6 weapon with no damage bonus.
D - 6(.5) per tier
Poison Spray.
Longsword attack with +2 bonus
C - 9 per tier
Casting eldritch blast on a target with hex
Attacking with a greatsword with a +2 bonus
B - 12 per tier
Eldritch Blast, hex, and Agonizing blast with +3 bonus
Attacking with a +2 greatsword with a +3 Str bonus.
Level 2 Magic Missile
A - 15 per tier
Eldritch blast, hex, agonizing blast with +6 Cha bonus
Attacking with a +3 greatsword with +5 Str Bonus.
S - 18 per tier
Attack with a +2 longbow, hunter's mark +3 Dex bonus, and Sharp Shooter.
Attacking with a +3 Greatsword, +5 Str, and raging for +3 damage.
Sneak attack+2d6 with +3 light crossbow and +5 Dex
Hitting 1 target with fireball - 28 damage.
So that's pretty good idea of how much damage per turn per tier is considered what grade (To me) keep in mind that at lower tiers, you can boost your damage per turn significantly with bonus actions. But they can't pickup as much slack in higher tier play as they don't scale up in damage as much as your standard actions.
For example Variant Human fighter with 16 Str at level 1 could pick Dueling fighting style and polearm master and uses a staff, Doing (d6+3+2)*2=17 damage a turn That is almost S grade damage, while holding a shield no less! But as he levels up, when he gets to tier 2, if he finds a +1 staff and gets 2 more Str he does (d6+4+2+1)*3=31.5 Damage, but thats only 15.75 damage per tier, so hes squarely at A grade damage. As staff fighter levels up, his bonus action attack is going to be a lower percentage of his overall damage per turn potential.
Keep in mind dealing damage is only one aspect of the game. Using vicious mockery is going to do F grade damage. BUT granting disadvantage on the targets next attack could save the day.