Mubarak(كبار) means happy. So, happy Eid Mubarak seems strange. It is possible to say Happy Al-Eid al-Mubarak, cus Eid is a noun and Mubarak is an adjective here and adding happy has no problems (in persian: Eidetun Mobarak! or Eid mobarak!). Of course Arabs know it is true or not, I'm Persian.