That is right before Enzo Maresca joined Chelsea team did not play well and had trouble finding their rhythm. But after Maresca arrived he made it priority to make team attack and defense stronger by bringing in new players. As coach Maresca probably focused on most important things and maybe he thought that if defense was strong goalkeeper would not have to worry as much. I think you are right that it is not fair to blame just one person. Every successful team needs time to develop and Maresca has started that process successfully. Also it is likely that coach has plan to find better goalkeeper slowly which will help team play even better.
Enzo Maresca did bring a good impact to the blues, but I feel Mauricio Pochettino is still better than Enzo, Mauricio Pochettino he gave a positive impact and more opportunities were not given by the Chelsea owner and also the players given were not in accordance with the wishes of the leader, this was also felt by Enzo, but apart from that I also prefer Mauricio Pochettino with his experience and also his playing style is more readable and I see that currently under Enzo it has not gone well.
The issue with Enzo Maresca is that he doesn't have the mentality that he can pull up good results from the kind of squad he is working with. The mentality is just too average, and it is one of the reasons why they have not won a serious team under his role as Chelsea's coach. he might have helped the team get good rhythm which has made Chelsea have a lot of possession in the games they have played, but that is just not enough. one thing with pochetinho is that it was clear he has a target he was looking out for, the boys where getting better and you could see the boys loved playing under him. Enzo came in with a two-team strategy, which worked for a while, and when injury came in, he is now struggling to get things right for them. If by the end of the season, he can't at least get a Champions League spot, all their effort at the start of the season will automatically.