According to my wife, who is French.
And according to my students, who are French.

The word lunatique is always an adjective and is alwost never used (in France) as a synonym for fou/folle (crazy).

It is never used as a noun (to designate a crazy person).

It always retains the idea of "lunar" "changing" "inconsistant" "unpredictable" etc.

According to the French thesaurus the following words are listed in the order of frequency as synonyms for "lunatique".

tordu (twisted)
as in twisted logic etc.

capricieux (capricious)

quinteux (changeable)

fantastique (as in fantasy)

bizarre (strange)

changeant (changing)

fou (crazy)

maniaque (irrational)