Lampard may not be the reason why they weren't bad, but he is certainly not a big help neither and I am expecting him not to be that great neither. This is why it is quite important to realize that he is not going to be helping them all that much neither, that's not how it works at all.
I personally believe that he is going to cause the team to be about the same as this season so they are going to sack him and get someone else. He was just the interim and he is not going to be staying for much longer. However, if by some miracle he stays and he is the manager for next season, I bet that they are not going to do that much better, not saying they will worse, they may do a little bit better, but not all that much, they need to be a lot better than what they did this season and Lampard can't make that happen.
It is already known that Pochettino will take over Chelsea. Lampard will be the co-manager as it seems. But I wouldn't be too harsh with Lampard. he is still a young guy and whether or not he can be a great coach still remains to be seen. Why do you think he will be a bad coach? Most of those newcomers need a good team to advance as a coach as well. There are a couple of examples like Luis Enrique who wouldn't have won the Champions League in 2015 thanks to coaching the Barcelona squad or Zidane winning it thanks to coaching a great Real Madrid squad. Lampard didn't really take control of a team that was in great shape. It was in shambles and he could only try to put the pieces together and save the season. That is why it's not quite fair to judge him just yet. He should also get a fair chance with a team that is functioning at least ok and not be a disaster from the start.