He's been fine in that he has done the borderline with a toxic squad where half of them want to leave and a dead season. I don't think you can entirely blame the manager here, but I'm not saying he's been good either. Said he was a let down. Just that I wouldn't say it's his fault.
He's not even doing the borderline at this point. I mean he got KOed from Fa Cup against a championship side, got KOed at home by Atletico in CL and has been languishing in 6th playing more games than the 4th and 5th teams. That's not the borderline or the bare minimum, especially for a manager getting the job in November or December. He had enough time to actually get something out of this season.
The problem is the football is also as bad as ever, the bad results would have been endured if there were signs of improvement on the pitch, but for Ralf there's simply not.
The previous managers lost the players after some period of time. Players were playing for LVG, Mourinho and Ole at the start. For Ralf it seems like he can't even get these players to do anything which is ultimately his problem.