i am a bbm student i want to get into iima am preparing for CAT .does work experience count or can i do pg right after bbm?

no there is no need to have work experience , you need good percentile in overall record in CAT test

You can do PG right after your BBM. Yes, work-experience has got some weightage in admissions, may be around 5 percent. Work-ex has following advantages 1. It enhances ones CV 2. It helps one develop an understanding of ones domain of interest whether it is finance or HR or Marketing etc.