I get cross about famous people getting a leg up in an unrelated field where they are not talented, because they are famous for something else.
I don’t think this is the best example of that though. Walliams’ books are genuinely popular with kids. A publisher might have published them even if they were by Joe Bloggs, because they are Dahl esque.
The reason I hate his books is not because of famous person leg up syndrome but because they are deeply unpleasant. DD was bought some by DM, I started to read one or two and took them away despite DD’s protests. DM has since had a look and agrees they are horrible.
If your DC likes them, have a read and see if you really want your DC to absorb those attitudes. Very sneery, snide, mean characters and dialogue - and that includes the “goodies” in the book. Roald Dahl had a lot of unpleasant characters too but they were the baddies, the main character and other goodies were always nice. Which has a very different effect on the reader IMO.
I am massively pro reading but I would genuinely rather my DD zoned out in front of the TV instead of reading Walliams.