The phrase 'was mere' is not a complete sentence and therefore is not correct in its present form.
However, the phrase does have different uses and can be used in written English if it is part of a longer sentence. For example: The task, which at first seemed impossible, was mere child's play after a few practice attempts.
Exact(57)
This was mere mannerism.
Presumably, this was mere electioneering.
This was mere distraction.
Any other approach was "mere philandering".
What came before yesterday was mere skirmishing.
All the rest was mere ostentation.
But this was mere "puffery"—harmless exaggeration.
It is possible, of course, that this was mere storytelling.
To many of them, the flag was mere cloth.
But all of that was mere conjecture last Friday night.
Plenty of that nose thumbing was mere gimmickry and gambolling.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com