They could only be used as a fleet in being, as the Allies would likely not know they weren't usable for lack of fuel, but it would easily prevent Operation Dragoon and potentially even Anzio. There would be a hell of a lot of extra caution in the Mediterranean, perhaps even heading off the Sardinia invasion, though no way it would effect the Sicily landings. Overall probably slows down the Mediterranean a fair bit and a lot of bombing effort is diverted from Italy and Germany to sink the French ships. It doesn't effect the outcome of the war, but is a pretty serious boon to the Germans for no added investment on their part. The French ships would be like what the Scharnhorst was in Brest for the RAF/RN, but much further away and a lot harder to hit; it would suck in a ton of effort to sink and divert attention from more serious targets, like Italian industry and perhaps even Austria. I mean look at how much effort the Brits put into sinking the Tirpitz, which was a useless hunk of steel that never did anything other than scare the Brits from port.