The phrase 'is simplicity' is correct and usable in written English.
You can use it when describing something that is uncomplicated and easy to understand. For example, "The key to success is simplicity."
Dictionary
is simplicity
noun
The quality or state of being simple, unmixed, or uncompounded; as, the simplicity of metals or of earths.
Exact(56)
Another virtue is simplicity.
The story is simplicity itself.
The device is simplicity itself.
The plot is simplicity itself.
Mr. Chao's gift is simplicity.
Setting up NetTalk is simplicity itself.
The app's interface is simplicity itself.
Actually, the answer is simplicity itself.
Similar(3)
It was simplicity itself.
It's simplicity itself.
"It was simplicity and perfection".
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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