눈치 has nothing to do with wit! 없으면Oblivious is way better. It's about understanding the atmosphere, and nothing to do with wit.
Never ask this insulting question!!!!!!!!!!!
Slow-witted (more commonly'stupid) isn't a compliment in English either and I can't imagine someone asking it unsarcastically. There may come a time you feel the need/want to ask/burn someone.
Yes, it's certainly rude. Oblivious is a good translation. "Why are you so unaware in social situations?"