The Blues slipped to a fourth straight Vanarama National League defeat and their seventh in their last nine matches in all competitions after a woeful first-half display on the 3G pitch at the Gallagher Stadium.
Relegation-threatened Maidstone were three goals up inside the first 14 minutes and added a fourth on the stroke of half-time.
Chester did mount a fightback of sorts after the break with top-scorer James Alabi notching his 11th goal in his last 13 league games from the penalty spot before defender Sam Hughes headed home to reduce the deficit further.
But McCarthy, who offered no excuses, admitted it had as much to do with the Stones letting their foot off the gas after a depressingly one-sided opening period.
What did Jon McCarthy say?
"The best I can do is apologise to the fans, especially the ones who have travelled all the way down here. It wasn't acceptable. it wasn't anywhere close to where we've been all season.
"The game was gone within 10 minutes, I'm bitterly disappointed. We've not been good lately, and we need to do some work and put that right.
"There are a load of excuses we can use - the pitch, the travel - but none of them really add up to me. It's actually about us and the frame of mind we got ourselves in.
"Tactically I'll accept some responsibility. I've worked hard at it this week. I've spent more time at it as I work harder when things aren't going well for us.
"But I haven't got it right. I accept that. It's my responsiblity. You can see when we got some better information to them at half-time they were more than capable of causing Maidstone a problem.
"We've got a hardworking group, who can't explain it, and I might not ever be able to explain it, even after I've watched the footage. But I'll do that on the way home, there's no respite now, and whatever I find was wrong with it, I'll put right."