You're much more likely to get an answer here if you explain in what sense you think the French army was "weak", and preferrably also where you get this notion from.

I am far from an expert on the subject and I hope others can follow up or correct me, but if by weak you are asking why they suffered setbacks in the manoeuvre period of the war can be put down to their view of the martial arts.

The French army still held a quite traditional view of warfare and fully expected their forces to form up on the battlefield across from their enemy in a fairly organised manned. This is expressed by their bright colourful uniforms at a time when the British had transferred to khaki for example.

I would suggest this has little to do with the French Army's​ fighting ability, or even their understanding that war had moved on, but is probably down to institutional issues. Militaries are usually very slow to adapt due to their tendancy to uphold tradition at almost all costs.