Mozart played mainly at the court of Queen Maria Theresa of Austria. But she had a daughter, Marie Antoinette, who was Queen of France (and who hosted Mozart on a tour).
It's unlikely that Mozart was "never" popular in France before World War II. What MAY have been true is that Mozart, who had his ups and downs, even after his death, was at a "low ebb" in France in 1940, and the occupation "revived" his popularity. While I don't know this for a fact, it's quite plausible that Mozart would have been unpopular in "Republican," anti-German France, basically between 1871-1940.
If Paxton was (basically) right about the latter, the 1940s revival of interest in Mozart would be a case of "Stockholm syndrome," defeated French identifying with Nazi conquerors.