small fry - people thought to be unimportant

... Fifteen years ago, when your average John Smith wanted to make a note to himself or write a letter, he pulled out his trusty pen and pad of paper. ...
http://www.andrews.edu/SM/2000-2001/Past/010902/xCorbin.html

A Brief Retrospective of the 21st Century
... For the average John Smith, the twentieth century began in optomism. The Sciences would obliterate ...
http://www.outlanders.fsnet.co.uk/tlh0205.htm
... But I would not hesitate to rate any average John Smith above a hundred of Petro Vasyliovychs for one simple reason: John Smith firmly, on the level of reflex ...
http://www.day.kiev.ua/DIGEST/1999/6/economy/eco-3.htm

This is Bradford | News
... Although he never suggested that the average John Smith should tackle armed robbers or knife-wielding muggers he clearly stated that it was everybody’s duty ...
http://www.thisisbradford.co.uk/bradford__district/ bradford/news/having-a-go.html

AnyBook4Less.com - ISBN: 0671742515 - LONG DARK TEA TIME OF THE ...
... But that's not what you buy it for. Then there's the business of the Norse
Gods walking the Earth, just like your average John Smith. Sort of. ...
http://www.anybook4less.com/detail/0671742515.html

nondescript - (n) a person is not easily classified and not very interesting
[adj] lacking distinct or individual characteristics; dull and uninteresting; "women dressed in nondescript clothes"; "a nondescript novel"

If he's just average, then he could be called (in the UK) "an average/ordinary bloke".
I assume you want a more pejorative term. Calling someone "a nonentity" (or "a nobody") is quite strongly pejorative, it means they have no special or interesting qualities.

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Phrases that give some Google hits:
just a nonentity
a complete nonentity

Russian term (edited): ���� ��

а если нужна именно идиома, то можно "a big zero". Если разговариваете с ньюйоркцем, можно "nebish". Это идиш, но в Нью-Йорке широко употребляется. Еще встречала " as dry as dust" , но не уверена.

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Note added at 2 hrs 37 mins (2003-07-31 13:50:57 GMT)
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вот еще : plain Jane ( похоже на другие варианты с Joe, только о женщине ), not much for looks; not much to look at. Больше смотри
http://www.m-w.com/cgi-bin/thesaurus

Etymology: Everyman, allegorical character in The Summoning of Everyman, 15th century English morality play
Date: 1906
: the typical or ordinary person

"An average Joe" is also OK

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Also:
\"Just a face in a crowd\" - this is for both a man and a woman

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Note added at 2003-07-31 14:52:54 (GMT)
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- i.e., there is nothing special about that person...

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Note added at 2003-08-01 00:36:01 (GMT)
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Also (somewhat different approach):
He/ She is nobody

(from Cambridge Advanced Learner's Dictionary)
wallflower (SHY PERSON) noun [C] INFORMAL
a shy person, especially a girl or woman, who is frightened to involve herself in social activities and does not attract much interest or attention:
Sooner or later someone would take pity on the poor wallflower and ask her to dance.

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Note added at 2003-07-31 22:36:12 (GMT)
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Other options:
mousy (if you turn it into an adjective)
doormat

there was even a movie "Simple men" by Hal Hartly which I translated. Can be considered as the US/(New-York)idiom, I guess. Good luck, Елена!