I guess you could call it undermining the judiciary, but not exactly usurping "democracy," since the FDR proposal would have required approval by Congress, which didn't happen, despite the fact that the Ds had the run of the place (indicative of how bad an idea it was).
One could argue that FDR's point with the court-packing proposal was that democracy was actually being undermined by the Court, because new laws enacted by a majority of the people's elected representatives were consistently being ruled unconstitutional in 5-4 decisions by conservative justices. Ironically, back in the 1930s the conservative jurists were the "activists."