Apparently he’s made over £100m from his books.
No, he hasn't.
The article states,
"David Walliams has joined a select group of authors to have sold more than £100 million worth of books."
£100m is the retail value of the books he's sold.
He'll be lucky to be paid a royalty of 8% on those sales. And a lot of those sales will be to places which demand high discounts from the publishers (supermarkets, for example, and book clubs that serve schools) which will have reduced his royalty on those sales by a considerable amount. It's still a lot of money, but it's a lot less than £100m.
On top of that, I don't know a single publisher which would make this announcement. It's almost certainly come from a publicist, and I'm not entirely sure that the numbers can be trusted.
The article goes on to say,
"According to The Bookseller, which published figures compiled by Nielsen Books, his forthcoming release The Beast of Buckingham Palace, could lead to the writer’s annual earnings topping £18 million for the first time. It said he had already earned £13 million this year, a 9 per cent rise against the same period last year."
A lot of that income comes from sources other than his writing: he'll earn repeat fees on Little Britain, for example, and he'll earn for personal appearances on TV.
I don't see a problem with his earning a lot of money if he's selling a lot of books: I am glad that someone as high-profile as him is popularising children's reading, and giving children something interesting and challenging to read.