Some brothers do not know that "Eeed Mubarak" is not an established greeting on the occasion of Eeed in the Sunnah.
Unfortunately, this greeting seems to be very popular amongst many Muslims on the occasion of Eeed.
To see the established method of greeting on Eeed please refer to CHAPTER EIGHTEEN of this article from TROID called "The Ruling Concering the Two 'Eeds from the Purified Sunnah".