I don’t think it’s made clear in the show that Tyrion is aware of her strength and intelligence at that point. After she finds out he has a woman stashed away and snags her, then he realizes she is a threat. Until that point, his attitude with her is somewhat condescending. She isn’t brilliant, because hey she snagged the wrong woman, but she’s smarter than most of the people she has to deal with. Westeros has a lot of dumbasses, for starters. But I don’t really agree that her intelligence and strength are so closely intertwined with cruelty and ruthlessness. They can be, but they aren’t always.