Looking at it again, there's a line that says "damage cut abilities (lit: ability A class) stack, but elemental resistance (from kamihime skills) doesn't". Even though the table is arranged like the other debuff tables where different classes stack but same classes don't...
As for Karin, fitting her into a Thunder team is going to be tough without sacrificing on debuffs. Thunder typically needs a debuff in every character slot to reach the caps, and the difference between -40% and -50% is pretty large.