The phrase "far less happy" is grammatically correct and can be used in written English.
Example: Despite her recent promotion, Mary felt far less happy in her new job than she had anticipated.
Investors were far less happy.
The effects of this have been far less happy.
Something equally liberating, but far less happy, happens to Jeanne.
But they were far less happy with the quality of teaching and school facilities.
Being happy matters but in many ways they are far less happy as a generation than their elders.
She is far less happy with political accountability for the new powers being granted to the commission and the Eurogroup of euro-zone governments.
In 1918, Marie Carmichael Stopes's best-selling "Married Love" included the lines "There is rottenness and danger at the foundations of the State if many of the marriages are unhappy.... To-day, particularly in the middle classes..., marriage is far less really happy than its surface appears".
But without meditation, he says, "I would probably be far less satisfied and happy.
People in Moscow smile far less than their happier, more expressive counterparts in Sao Paulo.
Saudi Arabia is far less trigger- or sword-happy.
