It's actually quite odd just how badly he is doing. It must be him. He must just be a deeply uninspiring man to attempt to lead a group.
I agree he must be a bad manager, but if I were to add a bit of nuance I would say he is also really bad at picking his jobs (and maybe somewhat unlucky). He did reasonable well at a lower level with Derby, partly because he could use his Chelsea connections to get some good young players on loan, but then he took the Chelsea job knowing fully how that usually turns out for a manager. Once the expectations were raised, when they bought Havertz, Werner, Chilwell, etc. he was shown for what he was - a manager with very little experience hired mostly based on his playing exploits. After that he accepted to manage an Everton side with bad ownership that was already flirting with relegation, and now taking over a Chelsea side in complete disarray. Chelsea needed an experienced manager to come in and steady the ship after they fired Potter, similar to what they have done in the past with Hiddink as a good example. Surely, Lampard could make that analysis.
I don't know if it's hubris, a sense of duty towards Chelsea, the need for a paycheck or all of the above, but taking the job in the first place was a bad mistake on his part as well. After he was fired in Chelsea the first time, he should have gone back down to the Championship level to work on his craft with less media attention. His reputation as a manager is in tatters now - much more than just a few months ago.