I think as far as the timeline goes, it's obviously before the events of all 3 Lupin III TV series. Because of the lack of overall plot, it's hard to say whether or not events from "A woman named Fujiko Mine" match up with all the events in the TV series without any ret-conning, maybe with the exception that Fujiko has memory loss.
There's never been any official announcement as it being a prequel and it's not consistent with the source material where the first meeting(s) between Lupin and Fujiko in the manga differ from this series (not that there's much consistency between the manga and any of the TV series to begin with).
Likewise, because of inconsistencies within the TV series itself (different characterizations of Lupin between the red and green jackets, different characterizations of his sidekicks and Fujiko) you could say the Fujiko series was a reboot just as likely as Lupin III, part 2 and part 3, the manga, were all reboots. But if you want to consider them all within the same continuity, I don't see any reason not to include Fujiko's series as a prequel, even if there would need to be some ret-conning (can easily just say Fujiko lost her memory again).