Theodore Sturgeon coined his law while editing a pulp science fiction magazine, but it is true more generally: "90% of everything is crap".
It's doubly true of BBC Sitcoms (95% is crap? 99% is crap?) because they commission mostly from their mates. The worst offender is Ben Elton: he hasn't written anything even remotely funny since Blackadder, but utter tosh by him is green-lighted on the basis of his name. But in the case of Mum (I haven't bothered to watch it, because of Sturgeon's law) it's yet another exhibit in the "why bother looking for writers more widely than the Cambridge Footlights, it worked for Python" catalogue. To take a concrete example of that tendency, John Cleese has never written a funny sitcom: Fawlty Towers is funny, but that's because Connie Booth wrote most of it. And yet even today, he's able to get commissioned by the BBC.
American networks produce serviceable comedy which is, at least, vaguely funny, because they have large writers rooms, and ruthlessly prune both shows and people when they don't deliver. The BBC in its "golden age" produced serviceable comedy because they had writers who had honed their craft writing for musical hall, or sketch shows. Galton and Simpson, say, or Norden and Muir, or Barry Cryer, or Eddie Braben, or the endless list of people who wrote for The Two Ronnies.
Now, since Monty Python, the BBC has been taken over by Cambridge graduates (note: that list of people who wrote The Two Ronnies and Morecombe and Wise and Hancock were all not Oxbridge graduates) who only employ other Cambridge graduates. It's at its worst at 6.30 on Radio 4, which is almost exclusively unfunny sub-Footlights detritus (has The News Quiz or The Now Show been funny in decades?) but the BBC1 and BBC2 schedules are full of it as well.