It's not Picard being deliberately insulting, it's just how a guy 300+ years from now would think of our own attire including what our soldiers wore.
No matter how well made the clothes are or what they represent, someone who is wearing garments appropriate for their job or their position will always be in uniform
. The person who is pretending to do so, as was Q, will always be in costume
. it has nothing to do with how Picard feels about the job represented by the uniform.