A person who tries to copy a professional and skillful person's act, without possessing any skills or experience needed
consider to ape someone TFD
to imitate or mimic, especially in a inept way.
It is pretty standard now for comedians to ape many politician's behavior and manner of speech.
Many of the synonyms for impostor, provided by Merriam-Webster.com, could work, though they are single words and not phrases or idioms:
charlatan, fake, faker, fakir, fraud, hoaxer, humbug, mountebank, phony (also phoney), pretender, quack, quacksalver, ringer, sham
M-W defines impostor as:
one that assumes false identity or title for the purpose of deception
I'd suggest charlatan:
one making usually showy pretenses to knowledge or ability
He is a rank amateur. According to The Word Detective:
a “rank amateur” is a person with absolutely no, zero, nada, zippo experience or expertise in a particular task or activity.
The word amateur has undergone a remarkable change in emphasis. The current meanings, given by Merriam Webster are:
one who engages in a pursuit, study, science, or sport as a pastime rather than as a profession.
one lacking in experience and competence in an art or science.
In an earlier era, definition 1 predominated, and amateurs were often more skilled than professionals. For example, in competitive figure skating, one had to maintain one's amateur standing to compete in official competitions, including the Olympics. When a skater "turned professional" it was often to join an ice show, where the skating was showier, but easier. The restrictions on what one could not do and still remain an amateur eased about the 1960s or so. (Not sure of exactly when.)
According to Etymonline, amateur originally meant
1784, "one who has a taste for some art, study, or pursuit, but does not practice it," from French amateur "one who loves, lover" (16c., restored from Old French ameour), from Latin amatorem
The same link gives the later, pejorative, meaning, citing amateurish (1863) as:
having the faults and deficiencies of a non-professional, 1863; from amateur + -ish.
I can't speak with any authority on why professionals came to be regarded as, and even actually to often be, more expert than amateurs. It is beyond the scope of this question, and an explanation involves social, economic and technological factors.
Rank in rank amateur intensifies the amateurishness and removes any possible ambiguity as to the meaning. For example, Edmund Hillary was an amateur climber when he climbed Everest, whereas many of the people who now pay tens of thousands of dollars to join an "expedition" with guides and Sherpas are rank amateurs as climbers.