.... uhh where does that SH thinks Karlach's unintelligent? From what conclusion did you arrive at that?
I actually misremembered the quote, it's a bit farfetched to assume that's what she meant. For context, it's: Shadowheart: We may need to increase our camp provisions now that we have Karlach along for the journey - those muscles don't run on fresh air, I'll wager. Still, she'll be useful in a fight.
More of a justification of bringing her along than anything else.