Really, Dude??? Hahaha
Okay... Even though your question comes across to me as a smart ass remark, I suspect there is a genuine question here. So, here are both sides of this statement...
One side is that Gracie refers to Luna as the "Smartest stupid girl" and in another part Luna refers to Gracie as the "Dumbest smart girl." What that means is this...
Luna is pretty ditzy and not the brightest light in the chandelier. She is not well read like Gracie and impulsive, making spontaneous decisions that aren't thought out at all. Luna is rather short on good old fashioned common sense as well. This leads to Gracie calling her stupid quite often. From a scholastic perspective, Gracie is not wrong. However, Luna has her bright moments and her specialties where she shines brightly such as her superior assassin's skills, stealth, knowledge of blades and firearms. I don't remember the exact context here but Luna obviously just had one of her bright, shining moments where one of Luna's strong points was just what they needed to solve a problem. Gracie, while astonished at Luna's momentary brilliance, still sees her sister as an idiot, hence the term "Smartest stupid girl." It is a left handed compliment aka a compliment wrapped around an insult.
Gracie is a genius with academic learning and strategic planning. She truly is the "mastermind" type who is always hatching a new brilliant scheme, usually aimed at getting something that Gracie wants. In general, Gracie is brilliant. However, Gracie also lacks common sense and social skills. She makes some pretty stupid mistakes as a result of those areas where she lacks knowledge and experience. One such blunder leads Luna to call Gracie the "Dumbest smart girl ever." Again, a left handed compliment.
I hope this helps as this is far from a new concept, my friend. Adventure ever on, Phat