He joins Benedict Cumberbatch as a critic of the show, after the Sherlock actor recently labelled it "f**king atrocious".
"As autumn approaches, soon it will be time for another series of Downton Abbey, and, frankly, if you think of the downward trajectory that show is on, I dread to think what it will be like," Mitchell begins in his most recent edition of Soapbox.
Explaining that he found the show's first series "mildly enjoyable", with "vaguely interesting characters experiencing vaguely plausible events in a vaguely interesting setting", he goes on: "Then the second series came along and it went completely nuts."
He describes it as "haemorrhaging history", referring to the fact that the series "whipped us through a whole sodding World War" while still attempting to "tell the personal stories... at the same speed as before".
"So in one show, you had that chauffeur... suggesting a hump to one of the Lord's daughters, and then in the next you get her huffy and slightly aroused reply; which would be fine, except in the story it's supposed to be eight months later," he explains.
"It was as if the First World War was taking place in Narnia."
He continues: "The plotting went nuts as well. Everything started happening to everybody. It crucially tipped over the line beyond which it's impossible to suspend your disbelief, and you can't help but realise it's just actors saying lies in hats."
Broadening his complaint, he concludes: "The thing that really worries me about this is that it didn't make a blind bit of difference to the show's success.
"The rising ratings for Downton Abbey suggest that properly scripting TV drama has about as much point to it as opening an art gallery for cattle."
Downton Abbey returns on Sunday (September 16) at 9pm on ITV1.