“O people the blessed month of Ramadan is approaching you and Allah (swt) has prescribed you to fast in this month.
“Ramadan Mubarak” is mostly said in Indian sub-continent, by the Indians and Pakistanis. Mubarak means “barakat” (blessing) as it is actually an Arabic word. But when used by Indians or Pakistanis, it is a type of congratulation.
Like someone passes an examination people congratulate by saying “Mubarak” in Urdu. So when Ramadan comes, they say “Ramadan Mubarak”. According to me its not a “bidah” (innovation).
It is good they are calling this month a blessed month. What word you choose is “mubah” (optional) like in Indian subcontinent people say “Ramadan Mubarak”, in gulf countries people call it “Ramadan Karim”. So all these words are good, holy month, blessed month.
So people should wish each other and remind each other of this blessed month.
(Source - Dr Zakir Naik Lecture)