I read that Mugdha Godse had to tone down her Marathi accent for the role in Fashion. What accent are bollywood stars expected to use? Is it the Dilli accent?
This is a very interesting question. I guess the answer to this question is the Delhi accent, or the general North Indian accent.
A more appropriate answer to this question is the diction should be "free of any accent", or local language influence. You may know that Marathi is a different language. Even some people native to Delhi may speak Hindi with a Punjabi, or Haryanvi accent. Speakers of other languages, like Bengali, Gujrati, Kashmiri, etc, bring their own accents to the language. So, the target is to speak it without an accent.
For example, what accent would a CNN newsreader be expected to have? They should be free from a Southern accent, or a New York accent, and more importantly, Italian or French accent, shouldn’t they?
However, in a movie, it would have made sense to make Mugdha speak in her own accent, which would have made her more real. There are other movies, where actors are made to speak in a regional accent.