Hi everybody!
There's actually not that many manga I can think of centered around outright quacks/witch doctors outside of a few super popular ones you've probably already heard of. There's actually not many medical manga period, and their protagonists tend to usually be prodigies of some sort. Unless you're willing to stretch the definition of "quack" to the protagonists of those dumb, medical-themed shonen action series, this is what I can suggest:
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Franken Fran's as close as you're gonna get to a proper quack, although even she's more whimsical than outright incompetent.
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Mushishi. I'll leave whether Ginko counts more as a pest exterminator or a witch doctor to you. Also competent.
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Black Jack. Degree of insanity of the cases depends on who's writing him. So competent he loops back into being a quack.