d&d tried hard to paint sansa as "she's the smartest person I know", as the person who knows Cercei better than anyone else and what Littlefinger can do. And they we have her lines like this: "That the woman (Cercei) who murdered my mother, father, and brother is dangerous?", were Sansa kind of forgets that it was Joffrey's order to execute Ned and that Freys and Boltons are responsible for the Red Wedding with Tywin their supporter and back up - who later tried to wash all the blame away from the Lannisters. That Sansa still trusts Littlefinger, though she understands he betrayed her - something even Littlefinger admits that he did. It's amazing to see how hard stansas believe in this "sansa = the smartest person" nonsense, but ignore that almost every Sansa's action or line shows otherwise.
Yeahhhhhh the issue with all of their show-only ideas is that they're shitty writers who couldn't even handle adapting the books, let alone coming up with their own material. They definitely wanted Sansa to be the smartest character ever but their incompetence got in the way. It also led to some truly baffling decisions, like writing Sansa being completely fooled by LF and having to go to Bran to realize he's lying to her. They genuinely couldn't come up with any scenarios to make her look competent so instead they cut the intelligence of every other character to try and make her look better. Keyword, try. That's basically a summation of everything that went wrong with the show though. They were more worried about her character then anyone else and the writing suffered for it.
