Learn English – n idiom/phrase that describes unskilled person in a skilled job

Well, the person you describe certainly lacks focus, so I guess you could say he is unfocused.

You asked, however, for a metaphor or phrase, so here are a few suggestions:

• He's certainly the dizziest person I know!

• His thinking is all jumbled up.

• Sally is certainly muddleheaded at this point in her job search.

• Rudy has turbidity of the brain pan.

• Martha suffers from excessive ideation.

• "The study of law is something new and unfamiliar to most of you, unlike any other schooling you have ever known before. You teach yourselves the law, but I train your minds. You come in here with a skull full of mush and, if you survive, you leave thinking like a lawyer" (Professor Kingsfield, from the opening narrative of the TV show "The Paper Chase")

• They say of George, "If he ever had an original thought, it would die of loneliness." They say of Frank, however, "Frank has so many original thoughts, his brain suffers from overcrowding"!

• Eddie has a whirling dervish in his head.

• If thoughts were sounds, he's have a cacophony in his head.

Shall I go on?

How about wrong tool for the job?

If you want to take out a screw, you choose a screwdriver, not a hammer. This doesn't mean hammers are "stupid". They're great at driving nails. They're just ill-equipped (not designed) for removing screws.

And so it is with people. You need to choose the right "tool" for the job. If you want someone to improve housekeeping, you choose a neatnik, not a slob (or so conventional wisdom would have it).

So, if Bob is the worst culprit when it comes to washing his own cup, and you make him responsible for ensuring that everyone washes their cup, I could say, with some justification, "You chose the wrong tool for the job."

Wrong tool for the job has the same connotations as "the fox guarding the henhouse": the fox is the wrong tool for the job. Perhaps this is why you thought of "the fox guarding the henhouse" when pondering your own question.