Actually it's not that Ishani doesn't apply her brain, the main problem is that she doesn't have brain. Every time she tries to apply her brain. First Chirag murder drama - She applied her GOBAR brain and thought rv killed chirag so changed the evidences at the murder scene. She can be again called brainless lady when her husband was shouting at top of his voice-Ishani I didn't kill chirag but if you have killed him I will take blame. This can be because of two reasons- Either she doesn't trust her husband or she has no brains.
She was mastermind behind bhoot plan so she applied her brain there but plan failed-why? Because ishani has Gobar in place of brain.
ML drama is plotted by ishani. She has applied lot of brain in it but it will backfire her very badly because she has a Gobar brain.
People with brain take intelligent decisions and don't use these cheap tricks like ML.