You go to the year before, when Lamar Jackson was literally playing MVP-caliber football and Brown's advanced stats still sucked:
58th in DYAR (just ahead of Christian Kirk)
63rd in DVOA (well behind Kirk and actually negative)
Plus it's not all on the QB. Rashad Bateman outpaced Brown in advanced stats on half as many targets (46th in DYAR and 39th in DVOA -- both positive).
Maybe. Brown spent a full quarter of Baltimore's offensive snaps on the sideline last year and more than one in five the year before. I don't think this changes much from last season's rotations; I expect that Brown will get 100% of Kirk's snaps and Moore gets stuck being told "We have to find a way to get him the ball more" when he gets 60 targets next year.
I have real doubts that Marquise Brown can play outside in a more traditional (HA!) NFL offense. Lamar doesn't like to throw down the field outside the hashes much, so Brown was running a lot of in-breaking routes on the snaps he was in the slot. If Brown is playing Kirk's role, he's a slot receiver.
I'm begging you, Cards Social Media, please stop trying to make "Bird City" happen.