That largely assumes that if there’s a problem with diminishing literacy these books are part of the solution not the cause.
I don’t think they’re the cause. Far from it.
The causes are more the power of retail giants like Tesco, Amazon - rise of TV, VOD, video games - all contributing to a culture where books and book selling aren’t valued.
If anything, mega sellers like Walliams are keeping publishers afloat in tough times.
I wish there was more variety in the mainstream. But it’s not Walliams fault that this is not the case.