My view is that his work is not really fan-fiction. Much fan fiction is an attempt to put someone's own views into an existing universe, usually using existing characters, or at least how I see it.
The Star Wars novels in a way could be thought of as sanctioned fanfic.
Jasper is using characters already in the public domain (with the exception of Mrs Tiggy-Winkle) and writing something completely different.
That said, I do not see anything inherently wrong with fanfic, provided no attempt is made to profit from it and acknowledgement to sources is given. On the other hand, if an author makes a request that people do not use their work in fanfic, this should be respected. And the whole practice does provide gems like CSI: Death by Chocolate
, which is a Charlie and the Chocolate Factory/CSI crossover that, bizarrely, works.
'We're all mad here. I'm mad, you're mad." [said the Cat.]
"How do you know I'm mad?" said Alice.
"You must be," said the Cat, "Or you wouldn't have come here."
- Lewis Carroll, Alice's Adventures In Wonderland