Worse, the French were extremely conflicted about actually engaging in direct combat with their former allies, very recently former allies. It’s difficult to bring a battle to a successful conclusion when half your officers and men would prefer to surrender and be interned rather than die as Nazi shills. In fact, the main battle took place partly as a matter of pride; several French forces in the Med surrendered in due course but the admiral in charge of that particular fleet felt insulted and slighted even though the Allies gave him several options of honorable surrender.
Also, as the above poster noted, some of these ships were in a very incomplete state in overhaul or refit, or never completed with full weaponry. Many of them were stuck in port, blockaded and without the resources to fight in the open ocean. Ships of that era were extremely vulnerable to (air) attack in tight seas, and worse if they were virtually immobile.
There are two more considerations. The first is that the French have been notoriously bad in warfare as compared to the technology level that they had ever since Trafalgar. This is because of a poor application of mobile warfare, political conflict, and worst for naval conflict, a lack of decisive action at critical times and timidity of strategy and tactics in the open ocean.
The other one is that the battleship you imagine would actually fight AGAINST the allies, not on their behalf. Vichy rules might include something like that, an immobile defensive battleship in SZ87-94 might be a fluffy addition.
Or you could see my “Free French” rules in the house rules forum and we could try to add a Vichy downside/FF upside somehow.
Tally Ho!