Ok so in terms of practicality some opponents/moves are definitely better than others. WALL OF TEXT INCOMING LOTS OF NUMBERS PEW PEW
You get meterless vortexes versus:
(OH/Low from my combos, might be better ones out there)
* Ermac 28%/23% - 1 bar 37%/35%
* Scorpion 29%/22% - 1 bar 37%/36%
* Mileena 27%/25% - 1 bar 37%/35%
* Quan Chi 31%/24% - 1 bar 38%/38%
* Jacqui 21%/16%
* Shinnok 17%/7% - 1 bar 32%/31%
You get 1 bar vortex on:
* Kotal Kahn 40%/40% - 32% OH meterless
* Kitana 31%/31% - 26% OH meterless
* Kung Lao 34%/31% - 25% OH meterless
* Erron Black 33%/29% (if you have space to whiff, but the range of his special sucks so it should be no problem)
* Kenshi: 27%/26% - Need to whiff which can be hard because of the range of his special. Can also get 18% meterless of the low.
* Sub Zero: 29%/24% - execution is weird though and you can get 20+% into vortex if you use it outside of mimicry setup.
* Sonya: 33%/29% whiff
You get high damage with a bar and a wrong guess on their side (30-40%) on:
* Kung Jin 40%/37%
* Kano 38%/35%
* Kenshi 36%/34%
* Liu Kang 36%/33%
* Johnny Cage 36%/34%
* Sonya 38%/35%
* Jax 39%/36%
* Cassie Cage 39%/36% - Can do 30% no bar off the overhead
* Raiden 38%/35% - Can do 27% no bar off the overhead
* D'vorah 35%/32%
* Jacqui 32%/30%
You get semi vortex with:
* Takeda - He gets 27% off of NJP and Low (also 27% if going for damage on OH).
You get guaranteed or chip options on:
* Kung Jin 11%
* Kenshi ~ 9% (8,66%) - Can reversal throw you
* Liu Kang -8,5% Can reversal throw you but that can be backdashed. HOWEVER if you backdash he could punish you with another reversal. Guessing game basically.
* Ferra/Torr 20%. It's a tick throw so can be armored/jumped. 15% guaranteed if you do it right away.
* Jax 9% Might be able to reversal throw. Also his command grabs might catch you but his fastest normal doesn't.
* Cassie Cage ~ 16% totally safe
* Goro 13% guaranteed
* D'vorah ~ 8% IIRC, might be more. Can be punished by grabbed I believe
Setup characters (1 bar):
* Reptile 33%/31% force ball pressure
* Jason 33%/31% Armor activation
Ok so looking at this list there are some obvious characters where Shinnok fares really well:
* Ermac, Scorpion, Mileena and Quan Chi all give you high damage meterless and high damage 1 bar vortexes.
* Kotal Kahn gets you massive damage and the option to keep the OH meterless
* Kitana and Kung Lao give nice vortexes where you can keep the OH meterless.
* Erron Black, Shinnok, Sub Zero, Kenshi, Sonya (possibly more) give you a nice chunk of damage and reset into vortex at the cost of a bar.
* Ferra/Torr, Cassie Cage and Goro all give you really nice, more or less guaranteed, damage.
* A lot of characters will lose 34-40% on a wrong guess after a mimicry. That is a lot for just one bar.
Some are not so good:
* Jacqui has a vortex but the damage is really low. Also she doesn't let you dish out that much damage either (30-32%)
* Jason is probably the worst. You will have to use the armor eventually to get your mimicry back and it has a huge downside so not sure if Impostor is worth it AT ALL vs Jason.
* Takeda has a vortex but it requires the NJP and it's not that high damage.
* Reptile can get some setups with force ball but nothing guaranteed.
TL;DR:
I think Mimicry is really worth it against a lot of characters for the huge damage you can get off a wrong guess and a bar. Some stolen moves aren't half bad either. Kano Ball, Liu Kangs kick, Raiden's superman, Jax's dash punch etc can all help you get in. Atm I think the only matchups where you might want to think twice about using Impostor is Jacqui, Takeda, Jason and Reptile.