Giving Machia a sedative was Midnight's idea and she asked Momo to do it.
Momo went along with it and she & the gang pulled through. Her part of the plan succeeded, which was "get the thing in his mouth".
Anything that happens afterwards is a miscalculation on Midnight's part.
For all we know, Momo made the most powerful sedative on earth, but if it's not gonna work (due to size maybe) then it's not gonna work, which is a mistake by Midnight.
But even then, there's NO fault for trying, because it was a sound plan in theory,
and as far as the actions needed to execute the plan..... they succeeded with it.
It's just that the outcome didn't fall favourably and he shrugged off the drug.
I would NOT consider anything that's happened a failure on Momo's part, based on the logic expressed in this post.
She executed (in actions at least) exactly what Midnight wanted her to do.
The outcome not being favourable after the fact... doesn't change that.
For me, Momo's plan being "worthless" only applies if they failed to get the drug in his mouth, since that was what they needed to do.
They actually managed to do it, which validates her more than it takes away from her.