From his first appearance its patently obvious that Dr. Zoidberg has no knowledge of human (or robot) anatomy. But his willingness to diagnose humans with Decapodian diseases shows he has some knowledge of Decapodian anatomy.

But has he ever been shown as a competent doctor of Decapods, or is he an al around terrible doctor?

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    The question @Richard mentions isn't exactly the same as yours, but its answers solve your question. Hence, voting to close. – Rand al'Thor Nov 5 '15 at 11:10
  • Not a dupe necessarily, I do seem to recall Professor Farnsworth saying he is an excellent doctor for aliens. – yondaime008 Nov 5 '15 at 11:10