Metaphor or idiom for "poor substitute"?

make do

Cambridge Dictionary:

make do ​

to manage to live without things that you would like to have or with things of a worse quality than you would like:

We didn't have cupboards so we made do with boxes.

You could use the term poor man's whatever, which means:

Cheaper than, simpler than, or inferior to.

Often used to describe a free or cheap alternative to a commodity.

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This is an excerpt from Wiktionary's "poor man's" entry and used under the CC-BY-SA 3.0 License.

This is sometimes used in the form of X is the poor man's Y. Here are some exemplary sentences from the Cambridge Idioms Dictionary, 2nd Ed.

He was only ever a mediocre singer - they used to call him 'the poor man's Frank Sinatra'.

'So what did you think of the film?' 'It was just a poor man's 'Pulp Fiction'.'

A sorry/pathetic/poor/sad excuse for something:

someone or something that is of very bad quality

Examples of usage:

• This is a sad excuse for a rental car.
• He's a pathetic excuse for a catcher.
• This is a poor excuse for a Pilsner.
• This is a sorry excuse for an answer.

Ersatz was used (as a noun) during wartime (certainly in North America, probably on the other side) to describe edibles (such as coffee made with chicory) that were poor replacements for what normally would be available.

How about knock-off?

knock-off: an unlicensed copy of something, especially fashion clothing, intended to be sold at a lower price than the original.

Knock-off items are poor substitutes for the originals.

This is a little bit out there, but I'll give it try.

I first heard the following saying from a colleague. When I tried to determine its origin a few minutes ago, I found numerous links and images, including one site that attributed it to UNKNOWN.

I'd rather choke on greatness than nibble on mediocrity.

I read this as meaning mediocrity is a poor substitute for greatness.

The phrase

That’s like bringing a knife to a gun fight.

refers to being inadequately prepared for a situation, or giving an inadequate response to a situation.  It is generally credited as being derived from a line in the movie The Untouchables.

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If you’re interested in similarly worded phrases that have radically different meanings (but not related to something being a poor substitute for something else), then you might enjoy this conversation from Lewis Carroll’s Alice’s Adventures in Wonderland:

“Then you should say what you mean,” the March Hare went on.

“I do,” Alice hastily replied; “at least — at least I mean what I say — that’s the same thing, you know.”

“Not the same thing a bit!” said the Hatter.  “You might just as well say that I see what I eat is the same thing as I eat what I see!”

“You might just as well say,” added the March Hare, “that I like what I get is the same thing as I get what I like!”

“You might just as well say,” added the Dormouse, who seemed to be talking in his sleep, “that I breathe when I sleep is the same thing as I sleep when I breathe!”

... which is discussed here and here.

Off the top of head I could come up with this—

not a patch on

greatly inferior to.

He no longer looked so handsome—he wasn't a patch on Peter.

Another good word is rubber stamp (alluding to the imprint-making property of rubber stamps, and could thus be extended metaphorically to signify unoriginality), and defined by Merriam-Webster as:

a mostly powerless yet officially recognized body or person that approves or endorses programs and policies initiated usually by a single specified source

an author who was ultimately just another rubber stamp of Hemingway.