This data is taken from the starcraft2coop website. It states that each Mutator has a number that represents its difficulty on a scale of 1-10, and that each tier of Brutal+ has a range representing the minimum and maximum numerical difficulty for that tier. At the bottom I wrote which Mutators I feel are off, feel free to dispute or list your own suggestions.
Brutal Difficulty — Minimum Difficulty — Maximum Difficulty
Brutal+ — 8 — 10
Brutal+2 — 11 — 12
Brutal+3 — 13 — 14
Brutal+4 — 15 — 16
Brutal+5 — 17 — 18
Brutal+6 — 19 — 20
Mutator Name — Numerical Difficulty
Aggressive Deployment — 3
Alien Incubation — 2
Avenger — 5
Barrier — 2
Black Death — 7
Blizzard — 4
Concussive Attacks — 1
Darkness — 2
Diffusion — 3
Double-edged — 3
Eminent Domain — 1
Evasive Maneuvers — 1
Fatal Attraction — 3
Fear — 3
Going Nuclear — 3
Hardened Will — 2
Heroes from the Storm — 10
Inspiration — 2
Just Die! — 7
Kill Bots — 6
Laser Drill — 2
Lava Burst — 3
Life Leech — 1
Long Range — 2
Mag-nificent — 4
Micro Transactions — 5
Mineral Shields — 2
Minesweeper — 6
Missile Command — 3
Moment of Silence — 2
Mutually Assured Destruction — 5
Orbital Strike — 1
Outbreak — 3
Photon Overload — 1
Polarity — 7
Power Overwhelming — 5
Propagators — 8
Purifier Beam — 2
Scorched Earth — 2
Self Destruction — 3
Shortsighted — 1
Slim Pickings — 5
Speed Freaks — 2
Temporal Field — 1
Time Warp — 1
Transmutation — 7
Twister — 2
Void Reanimators — 5
Void Rifts — 10
Walking Infested — 2
We Move Unseen — 3
Mutators I think could be higher: Avenger, Black Death, Diffusion, Fatal Attraction, Fear, Going Nuclear, Hardened Will, Inspiration, Kill Bots, Mineral Shields, Minesweeper, Missile Command, Moment of Silence, Orbital Strike, Polarity, Scorched Earth, Self Destruction, Slim Pickings, Transmutation, Void Reanimators
Mutators I think could be lower: Blizzard, Micro Transactions, Mutually Assured Destruction
I’d like to point out that these are for Brutal+ and NOT for the Weekly Mutation. Knowing what Mutators you are getting ahead of time before choosing your Commander effectively lowers the difficulty of them. Also most of my suggestions are intended for only 1 or 2 point adjustments, with maybe a couple exceptions.