If your whipped cream is stiff or even overwhipped when you try to incorporate it into the ganach, little blobs of cream won’t want to incorporate into the ganache. This means your mousse might have either little lumps of unincorporated cream in your mousse or you will overwork it and end up with mousse that is not fluffy. If you realize your whipped cream has reached stiff peaks by accident, stir in a couple of tablespoons of additional unwhipped cream or soy milk to the whipped cream until you reach a medium-peak whipped cream before folding it into the ganache.