In a TV interview, the producer of this song Quincy Jones was asked by Mary J Blige who was MJ talking about in Dirty Diana, and Quincy said: "I thought it was about Diana Ross, like everyone else."
Quincy himself was aware that everyone thought it was about her and he didn't object to this assumption, so I'm guessing that it's true.
In a TV interview, the producer of this song Quincy Jones was asked by Mary J Blige who was MJ talking about in Dirty Diana, and Quincy said: "I thought it was about Diana Ross, like everyone else."
Quincy himself was aware that everyone thought it was about her and he didn't object to this assumption, so I'm guessing that it's true.