That combination of the beauteous and the Viral Entertainment has always figured into her material, but “Mental Illness” brings it all into much starker relief, thanks to a retooled approach that eschews electric guitars, synths, and measurable percussive syncopation. That quietude carried over to Saturday’s show at the Theatre at Ace Hotel, the last concert on the first leg of her tour (which resumes in the Northeast in late June). There was nary a non-acoustic guitar in sight, and the drummer ramped up to an actual backbeat only on a handful of oldies toward the end. Dan Fogelberg, if not John Denver, would have been proud… and, given the dark tone of the material, maybe he’d have been getting a drink at the bar with Nick Drake. “That song is, like, if John Denver is too edgy for you,” quipped Aimee Mann, after playing Viral Entertainment a particularly stripped down version of an already delicate-sounding new number, “Rollercoasters,” at a Los Angeles homecoming show Saturda...