The way I see it, what makes acidic so good is his almost parodoxical design
His zoning requires patience to get through and very slow movement (don't expect to run in on acidic), yet patience is exactly what loses you 20-30% of your health against acidic. You're forced to approach acidic making dangerous reads, and losing lots of health
If there was no run button id say acidic would be better, but run in and press buttons > zoning by far in MKX.
I guarantee you won't be running in on sonicfox as easily as doubletapping forward + r2
As ATP said, I can't see how one could be definitively better than the other, it's based on way too many variables:
Patience of the opposing PLAYER
Mobility of the opposing CHARACTER
Design of opposing character
Luck based on reads (acidic), luck based on guessing mixups (tarkatan)
Also I find it strange that you say Tarkatan has better footsies, as I'd disagree and say there about the same WHILE acidic has better overall
spacing because his acid restricts your movement so much