This is not really my specialty, so I cannot comment in depth on the topic. But, it's been twelve hours and you haven't gotten any responses, so I'll try to help you out.

The French ships at Mers-el-Kebir would certainly have been significant to the German Navy, as would any of the other elements of the French fleet that the Germans tried to acquire, such as at Toulon. They're big, expensive, difficult-to-manufacture pieces of military hardware and that cannot be discounted.

On the other hand, the Kriegsmarine was pretty focused on one strategic goal, the disruption of the trans-Atlantic commerce that supplied Britain's caloric and material needs. I've never heard of them seriously considering major fleet actions against the Royal Navy, and even their biggest ships, like the Bismark, were pretty much just commerce raiders. This focus on commerce explains their preoccupation with submarines. So, a few battlecruisers and destroyers or other surface warships in the Mediterranean might have helped the Germans to an extent, but it's hard to see them as a game changer. This is particularly the case when you consider what the British did to the Italian fleet at Taranto. Would a few more French ships have helped? Sure. Would they have changed anything? It seems doubtful.