Hmmm, two thoughts:
You could try dissolving sugar in a little of the jam/preserves by heating them together. If all the sugar dissolves and it’s smooth, you could then add it to the mousseline when cool. But there is a limit to how much extra jam the mousseline will hold.
The other thought is to leave it as is, and make sure the chocolate frosting is sweet enough. Since the chocolate is the part of the cake that might be construed as bitter, perhaps that is the most important component to get extra sweet.
OK, must go now, all this talk of extra sweetness…