In general, criticising your players in public is a sign of desperation, a last resort when all else has failed in your attempts to motivate them. The problem here is the whole team has been playing poorly this season, Johnson has been poor but significantly worse than anyone else? I'm not so sure about that. Not to mention the fact he's been out injured and is injured currently so can't even react on the pitch to his manager's comments.
The overarching issue which can't be ignored is how Hodgson's tactical approach simply doesn't benefit attacking full backs - Do you think Alves or Evra would be flourishing in this team at the moment? Johnson would have had a field day against Blackburn because the team was on the front foot. Watch the video MilanKakaBaros posted in this thread of the MOTD montage of Johnson against Stoke last season, look how far forward he is in most of the clips - our full backs barely get over the halfway line under Hodgson away from home and that's not even an exaggeration.
I detest the route we're going down with Hodgson, absolutely hate it, I want a Liverpool team to have a modern tactical approach, play some variation on a 4-2-3-1 with technical midfielders and athletic attacking full backs, always on the front foot, pressing the ball high up the pitch but the reality is if we are going to persist with achieving Hodgson's vision we might as well sell Johnson for his benefit and ours, buy a more defensive solid full back and spend the cash on a second striker so we can play a genuine 4-4-2. We're a mish-mash of different styles the classic square pegs in round holes.
For Hodgson to criticise publicly a player who's not been our worst player, who's been out injured much of the time, who's currently out injured so can't even react and who's poor form is in no small part down to his own tactical approach is frankly typical Hodgson - he's completely out of his depth.