Same as what others say above. I'd consider caramelizing some sugar or using Belgian Candi sugar syrup rather than using actual caramel. Actual caramel contains cream and/or butter, and the fats in those could give you some rank off-tastes as fermentation goes along. Caramelized sugar (or even plain brown sugar) plus a little lactose in late-boil would give you the combination of toastiness and sweetness that I suspect you're looking for in a caramel mocha stout.
If you're looking to cultivate the buttery taste that's present in caramel, you could consider deliberately producing diacetyl, but that's a dangerous game, friend.