"is not fun" is a grammatically correct and usable phrase in written English.
It is often used to express that something is not enjoyable or entertaining. Example: Going to the DMV is not fun, but it is necessary to renew your driver's license.
Exact(59)
"This is not fun.
Losing is not fun.
Traffic is not fun.
Being infamous is not fun.
This is not fun time.
Implant surgery is not fun.
Playing Mississippi is not fun.
A president's life is not fun.
Unpicking that stuff is not fun.
This is not fun and games.
Talking about HIV is not fun.
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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